diff --git a/appinfo/routes.php b/appinfo/routes.php
index 22074e93878..f8e7c614559 100644
--- a/appinfo/routes.php
+++ b/appinfo/routes.php
@@ -33,6 +33,11 @@
 			'url' => '/not-found',
 			'verb' => 'GET',
 		],
+		[
+			'name' => 'Page#duplicateSession',
+			'url' => '/duplicate-session',
+			'verb' => 'GET',
+		],
 
 		[
 			'name' => 'Page#showCall',
diff --git a/docs/participant.md b/docs/participant.md
index 3482581f498..bb72dd0e42c 100644
--- a/docs/participant.md
+++ b/docs/participant.md
@@ -109,18 +109,28 @@ Base endpoint is: `/ocs/v2.php/apps/spreed/api/v1`
     field | type | Description
     ------|------|------------
     `password` | string | Optional: Password is only required for users which are of type `4` or `5` and only when the conversation has `hasPassword` set to true.
+    `force` | bool | If set to `false` and the user has an active session already a `409 Conflict` will be returned (Default: true - to keep the old behaviour)
 
 * Response:
     - Status code:
         + `200 OK`
         + `403 Forbidden` When the password is required and didn't match
         + `404 Not Found` When the conversation could not be found for the participant
+        + `409 Conflict` When the user already has an active session in the conversation. The suggested behaviour is to ask the user whether they want to kill the old session and force join unless the last ping is older than 60 seconds or older than 40 seconds when the conflicting session is not marked as in a call.
 
-    - Data:
+    - Data in case of `200 OK`:
+
+        field | type | Description
+        ------|------|------------
+        `sessionId` | string | 512 character long string
+
+    - Data in case of `409 Conflict`:
 
         field | type | Description
         ------|------|------------
         `sessionId` | string | 512 character long string
+        `inCall` | int | Flags whether the conflicting session is in a potential call
+        `lastPing` | int | Timestamp of the last ping of the conflicting session
 
 ## Leave a conversation (not available for call and chat anymore)
 
diff --git a/lib/Controller/PageController.php b/lib/Controller/PageController.php
index b75843c5543..d3246e1d06e 100644
--- a/lib/Controller/PageController.php
+++ b/lib/Controller/PageController.php
@@ -152,7 +152,17 @@ public function authenticatePassword(string $token, string $password = ''): Resp
 	 * @return Response
 	 */
 	public function notFound(): Response {
-		return new RedirectResponse($this->url->linkToRouteAbsolute('spreed.Page.index'));
+		return $this->index();
+	}
+
+	/**
+	 * @PublicPage
+	 * @NoCSRFRequired
+	 *
+	 * @return Response
+	 */
+	public function duplicateSession(): Response {
+		return $this->index();
 	}
 
 	/**
diff --git a/lib/Controller/RoomController.php b/lib/Controller/RoomController.php
index ae071a0287a..8e9ee6fc5c1 100644
--- a/lib/Controller/RoomController.php
+++ b/lib/Controller/RoomController.php
@@ -1159,15 +1159,51 @@ public function setPassword(string $password): DataResponse {
 	 *
 	 * @param string $token
 	 * @param string $password
+	 * @param bool $force
 	 * @return DataResponse
 	 */
-	public function joinRoom(string $token, string $password = ''): DataResponse {
+	public function joinRoom(string $token, string $password = '', bool $force = true): DataResponse {
 		try {
 			$room = $this->manager->getRoomForParticipantByToken($token, $this->userId);
 		} catch (RoomNotFoundException $e) {
 			return new DataResponse([], Http::STATUS_NOT_FOUND);
 		}
 
+		$previousSession = null;
+		if ($this->userId !== null) {
+			try {
+				$previousSession = $room->getParticipant($this->userId);
+			} catch (ParticipantNotFoundException $e) {
+			}
+		} else {
+			$session = $this->session->getSessionForRoom($token);
+			try {
+				$previousSession = $room->getParticipantBySession($session);
+			} catch (ParticipantNotFoundException $e) {
+			}
+		}
+
+		if ($previousSession instanceof Participant && $previousSession->getSessionId() !== '0') {
+			// Previous session was active
+			if ($force === false) {
+				return new DataResponse([
+					'sessionId' => $previousSession->getSessionId(),
+					'inCall' => $previousSession->getInCallFlags(),
+					'lastPing' => $previousSession->getLastPing(),
+				], Http::STATUS_CONFLICT);
+			}
+
+			if ($previousSession->getInCallFlags() !== Participant::FLAG_DISCONNECTED) {
+				$room->changeInCall($previousSession, Participant::FLAG_DISCONNECTED);
+			}
+
+			if ($this->userId === null) {
+				$room->removeParticipantBySession($previousSession, Room::PARTICIPANT_LEFT);
+			} else {
+				$room->leaveRoomAsParticipant($previousSession);
+			}
+		}
+
 		$user = $this->userManager->get($this->userId);
 		try {
 			$result = $room->verifyPassword((string) $this->session->getPasswordForRoom($token));
@@ -1209,7 +1245,7 @@ public function leaveRoom(string $token): DataResponse {
 				$room->removeParticipantBySession($participant, Room::PARTICIPANT_LEFT);
 			} else {
 				$participant = $room->getParticipant($this->userId);
-				$room->leaveRoom($participant->getUser());
+				$room->leaveRoomAsParticipant($participant);
 			}
 		} catch (RoomNotFoundException $e) {
 		} catch (ParticipantNotFoundException $e) {
diff --git a/lib/Controller/SignalingController.php b/lib/Controller/SignalingController.php
index 4070f72973b..6c3dc06cdad 100644
--- a/lib/Controller/SignalingController.php
+++ b/lib/Controller/SignalingController.php
@@ -313,7 +313,25 @@ public function pullMessages(string $token): DataResponse {
 			$data[] = ['type' => 'usersInRoom', 'data' => $this->getUsersInRoom($room, $pingTimestamp)];
 		} catch (RoomNotFoundException $e) {
 			$data[] = ['type' => 'usersInRoom', 'data' => []];
-			return new DataResponse($data, Http::STATUS_NOT_FOUND);
+
+			// Was the session killed or the complete conversation?
+			try {
+				$room = $this->manager->getRoomForParticipantByToken($token, $this->userId);
+				if ($this->userId) {
+					// For logged in users we check if they are still part of the public conversation,
+					// if not they were removed instead of having a conflict.
+					$room->getParticipant($this->userId);
+				}
+
+				// Session was killed, make the UI redirect to an error
+				return new DataResponse($data, Http::STATUS_CONFLICT);
+			} catch (ParticipantNotFoundException $e) {
+				// User removed from conversation, bye!
+				return new DataResponse($data, Http::STATUS_NOT_FOUND);
+			} catch (RoomNotFoundException $e) {
+				// Complete conversation was killed, bye!
+				return new DataResponse($data, Http::STATUS_NOT_FOUND);
+			}
 		}
 
 		return new DataResponse($data);
diff --git a/lib/Room.php b/lib/Room.php
index 9924241af7d..ca02918e804 100644
--- a/lib/Room.php
+++ b/lib/Room.php
@@ -839,6 +839,14 @@ public function leaveRoom(string $userId, ?string $sessionId = null): void {
 			return;
 		}
 
+		$this->leaveRoomAsParticipant($participant, $sessionId);
+	}
+
+	/**
+	 * @param Participant $participant
+	 * @param string|null $sessionId
+	 */
+	public function leaveRoomAsParticipant(Participant $participant, ?string $sessionId = null): void {
 		$event = new ParticipantEvent($this, $participant);
 		$this->dispatcher->dispatch(self::EVENT_BEFORE_ROOM_DISCONNECT, $event);
 
@@ -847,22 +855,26 @@ public function leaveRoom(string $userId, ?string $sessionId = null): void {
 		$query->update('talk_participants')
 			->set('session_id', $query->createNamedParameter('0'))
 			->set('in_call', $query->createNamedParameter(0, IQueryBuilder::PARAM_INT))
-			->where($query->expr()->eq('user_id', $query->createNamedParameter($userId)))
+			->where($query->expr()->eq('user_id', $query->createNamedParameter($participant->getUser())))
 			->andWhere($query->expr()->eq('room_id', $query->createNamedParameter($this->getId(), IQueryBuilder::PARAM_INT)))
 			->andWhere($query->expr()->neq('participant_type', $query->createNamedParameter(Participant::USER_SELF_JOINED, IQueryBuilder::PARAM_INT)));
-		if (!empty($sessionId)) {
+		if ($sessionId !== null && $sessionId !== '0') {
 			$query->andWhere($query->expr()->eq('session_id', $query->createNamedParameter($sessionId)));
+		} elseif ($participant->getSessionId() !== '0') {
+			$query->andWhere($query->expr()->eq('session_id', $query->createNamedParameter($participant->getSessionId())));
 		}
 		$query->execute();
 
 		// And kill session when leaving a self joined room
 		$query = $this->db->getQueryBuilder();
 		$query->delete('talk_participants')
-			->where($query->expr()->eq('user_id', $query->createNamedParameter($userId)))
+			->where($query->expr()->eq('user_id', $query->createNamedParameter($participant->getUser())))
 			->andWhere($query->expr()->eq('room_id', $query->createNamedParameter($this->getId(), IQueryBuilder::PARAM_INT)))
 			->andWhere($query->expr()->eq('participant_type', $query->createNamedParameter(Participant::USER_SELF_JOINED, IQueryBuilder::PARAM_INT)));
-		if (!empty($sessionId)) {
+		if ($sessionId !== null && $sessionId !== '0') {
 			$query->andWhere($query->expr()->eq('session_id', $query->createNamedParameter($sessionId)));
+		} elseif ($participant->getSessionId() !== '0') {
+			$query->andWhere($query->expr()->eq('session_id', $query->createNamedParameter($participant->getSessionId())));
 		}
 		$query->execute();
 
diff --git a/package-lock.json b/package-lock.json
index a5121ec4c10..6ba71eda3ca 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -2560,6 +2560,21 @@
 				}
 			}
 		},
+		"@nextcloud/browser-storage": {
+			"version": "0.1.1",
+			"resolved": "https://registry.npmjs.org/@nextcloud/browser-storage/-/browser-storage-0.1.1.tgz",
+			"integrity": "sha512-bWzs/A44rEK8b3CMOFw0ZhsenagrWdsB902LOEwmlMCcFysiFgWiOPbF4/0/ODlOYjvPrO02wf6RigWtb8P+gA==",
+			"requires": {
+				"core-js": "3.6.1"
+			},
+			"dependencies": {
+				"core-js": {
+					"version": "3.6.1",
+					"resolved": "https://registry.npmjs.org/core-js/-/core-js-3.6.1.tgz",
+					"integrity": "sha512-186WjSik2iTGfDjfdCZAxv2ormxtKgemjC3SI6PL31qOA0j5LhTDVjHChccoc7brwLvpvLPiMyRlcO88C4l1QQ=="
+				}
+			}
+		},
 		"@nextcloud/browserslist-config": {
 			"version": "1.0.0",
 			"resolved": "https://registry.npmjs.org/@nextcloud/browserslist-config/-/browserslist-config-1.0.0.tgz",
diff --git a/package.json b/package.json
index c8f617d6e3f..0c3a6de282f 100644
--- a/package.json
+++ b/package.json
@@ -19,6 +19,7 @@
 		"@juliushaertl/vue-richtext": "^0.3.1",
 		"@nextcloud/auth": "^1.2.3",
 		"@nextcloud/axios": "^1.3.2",
+		"@nextcloud/browser-storage": "^0.1.1",
 		"@nextcloud/dialogs": "^1.3.0",
 		"@nextcloud/event-bus": "^1.1.4",
 		"@nextcloud/initial-state": "^1.1.2",
diff --git a/src/App.vue b/src/App.vue
index 943839506b5..68ca324ccba 100644
--- a/src/App.vue
+++ b/src/App.vue
@@ -31,7 +31,7 @@
 		</AppContent>
 		<RightSidebar
 			:show-chat-in-sidebar="isInCall" />
-		<PreventUnload :when="isInCall" />
+		<PreventUnload :when="warnLeaving" />
 	</Content>
 </template>
 
@@ -56,6 +56,8 @@ import {
 } from './utils/webrtc/index'
 import { emit } from '@nextcloud/event-bus'
 import browserCheck from './mixins/browserCheck'
+import duplicateSessionHandler from './mixins/duplicateSessionHandler'
+import isInCall from './mixins/isInCall'
 import talkHashCheck from './mixins/talkHashCheck'
 import { generateUrl } from '@nextcloud/router'
 
@@ -72,6 +74,8 @@ export default {
 	mixins: [
 		browserCheck,
 		talkHashCheck,
+		duplicateSessionHandler,
+		isInCall,
 	],
 
 	data: function() {
@@ -112,8 +116,8 @@ export default {
 			}
 		},
 
-		isInCall() {
-			return this.participant.inCall !== PARTICIPANT.CALL_FLAG.DISCONNECTED
+		warnLeaving() {
+			return !this.isLeavingAfterSessionConflict && this.isInCall
 		},
 
 		/**
@@ -220,7 +224,9 @@ export default {
 				// We have to do this synchronously, because in unload and beforeunload
 				// Promises, async and await are prohibited.
 				signalingKill()
-				leaveConversationSync(this.token)
+				if (!this.isLeavingAfterSessionConflict) {
+					leaveConversationSync(this.token)
+				}
 			}
 		})
 
diff --git a/src/FilesSidebarCallViewApp.vue b/src/FilesSidebarCallViewApp.vue
index fc1af2a2551..7ad6202810b 100644
--- a/src/FilesSidebarCallViewApp.vue
+++ b/src/FilesSidebarCallViewApp.vue
@@ -23,10 +23,10 @@
 <template>
 	<div v-if="isInFile">
 		<CallView
-			v-show="isInCall"
+			v-show="showCallView"
 			:token="token"
 			:is-sidebar="true" />
-		<PreventUnload :when="isInCall" />
+		<PreventUnload :when="warnLeaving" />
 	</div>
 </template>
 
@@ -35,6 +35,8 @@ import { PARTICIPANT } from './constants'
 import CallView from './components/CallView/CallView'
 import PreventUnload from 'vue-prevent-unload'
 import browserCheck from './mixins/browserCheck'
+import duplicateSessionHandler from './mixins/duplicateSessionHandler'
+import isInCall from './mixins/isInCall'
 import talkHashCheck from './mixins/talkHashCheck'
 
 export default {
@@ -48,6 +50,8 @@ export default {
 
 	mixins: [
 		browserCheck,
+		duplicateSessionHandler,
+		isInCall,
 		talkHashCheck,
 	],
 
@@ -90,34 +94,38 @@ export default {
 		 *          otherwise.
 		 */
 		isInFile() {
-			if (this.fileId !== this.fileIdForToken) {
-				return false
-			}
-
-			return true
+			return this.fileId === this.fileIdForToken
 		},
 
-		isInCall() {
+		showCallView() {
 			// FIXME Remove participants as soon as the file changes so this
 			// condition is not needed.
 			if (!this.isInFile) {
 				return false
 			}
 
+			return this.isInCall
+		},
+
+		participant() {
 			const participantIndex = this.$store.getters.getParticipantIndex(this.token, this.$store.getters.getParticipantIdentifier())
 			if (participantIndex === -1) {
-				return false
+				return {
+					inCall: PARTICIPANT.CALL_FLAG.DISCONNECTED,
+				}
 			}
 
-			const participant = this.$store.getters.getParticipant(this.token, participantIndex)
+			return this.$store.getters.getParticipant(this.token, participantIndex)
+		},
 
-			return participant.inCall !== PARTICIPANT.CALL_FLAG.DISCONNECTED
+		warnLeaving() {
+			return !this.isLeavingAfterSessionConflict && this.showCallView
 		},
 	},
 
 	watch: {
-		isInCall: function(isInCall) {
-			if (isInCall) {
+		showCallView: function(showCallView) {
+			if (showCallView) {
 				this.replaceSidebarHeaderContentsWithCallView()
 			} else {
 				this.restoreSidebarHeaderContents()
diff --git a/src/FilesSidebarTabApp.vue b/src/FilesSidebarTabApp.vue
index e2ee8b8e8bd..8e9ffd73885 100644
--- a/src/FilesSidebarTabApp.vue
+++ b/src/FilesSidebarTabApp.vue
@@ -62,6 +62,7 @@ import { loadState } from '@nextcloud/initial-state'
 import Axios from '@nextcloud/axios'
 import CallButton from './components/TopBar/CallButton'
 import ChatView from './components/ChatView'
+import duplicateSessionHandler from './mixins/duplicateSessionHandler'
 
 export default {
 
@@ -72,6 +73,10 @@ export default {
 		ChatView,
 	},
 
+	mixins: [
+		duplicateSessionHandler,
+	],
+
 	data() {
 		return {
 			// needed for reactivity
@@ -151,7 +156,9 @@ export default {
 				// We have to do this synchronously, because in unload and beforeunload
 				// Promises, async and await are prohibited.
 				signalingKill()
-				leaveConversationSync(this.token)
+				if (!this.isLeavingAfterSessionConflict) {
+					leaveConversationSync(this.token)
+				}
 			}
 		})
 	},
diff --git a/src/PublicShareAuthSidebar.vue b/src/PublicShareAuthSidebar.vue
index fd46d399d3c..c4ad49552ad 100644
--- a/src/PublicShareAuthSidebar.vue
+++ b/src/PublicShareAuthSidebar.vue
@@ -49,6 +49,7 @@ import {
 } from './services/participantsService'
 import { signalingKill } from './utils/webrtc/index'
 import browserCheck from './mixins/browserCheck'
+import duplicateSessionHandler from './mixins/duplicateSessionHandler'
 import talkHashCheck from './mixins/talkHashCheck'
 
 export default {
@@ -62,6 +63,7 @@ export default {
 
 	mixins: [
 		browserCheck,
+		duplicateSessionHandler,
 		talkHashCheck,
 	],
 
@@ -107,7 +109,9 @@ export default {
 				// We have to do this synchronously, because in unload and beforeunload
 				// Promises, async and await are prohibited.
 				signalingKill()
-				leaveConversationSync(this.token)
+				if (!this.isLeavingAfterSessionConflict) {
+					leaveConversationSync(this.token)
+				}
 			}
 		})
 	},
diff --git a/src/PublicShareSidebar.vue b/src/PublicShareSidebar.vue
index 9edb2377afb..13d2bde1300 100644
--- a/src/PublicShareSidebar.vue
+++ b/src/PublicShareSidebar.vue
@@ -35,7 +35,7 @@
 				<CallView v-if="isInCall"
 					:token="token"
 					:is-sidebar="true" />
-				<PreventUnload :when="isInCall" />
+				<PreventUnload :when="warnLeaving" />
 				<CallButton class="call-button" />
 				<ChatView :token="token" />
 			</template>
@@ -59,6 +59,8 @@ import {
 } from './services/participantsService'
 import { signalingKill } from './utils/webrtc/index'
 import browserCheck from './mixins/browserCheck'
+import duplicateSessionHandler from './mixins/duplicateSessionHandler'
+import isInCall from './mixins/isInCall'
 import talkHashCheck from './mixins/talkHashCheck'
 
 export default {
@@ -74,6 +76,8 @@ export default {
 
 	mixins: [
 		browserCheck,
+		duplicateSessionHandler,
+		isInCall,
 		talkHashCheck,
 	],
 
@@ -109,15 +113,19 @@ export default {
 			return this.state.isOpen
 		},
 
-		isInCall() {
+		participant() {
 			const participantIndex = this.$store.getters.getParticipantIndex(this.token, this.$store.getters.getParticipantIdentifier())
 			if (participantIndex === -1) {
-				return false
+				return {
+					inCall: PARTICIPANT.CALL_FLAG.DISCONNECTED,
+				}
 			}
 
-			const participant = this.$store.getters.getParticipant(this.token, participantIndex)
+			return this.$store.getters.getParticipant(this.token, participantIndex)
+		},
 
-			return participant.inCall !== PARTICIPANT.CALL_FLAG.DISCONNECTED
+		warnLeaving() {
+			return !this.isLeavingAfterSessionConflict && this.isInCall
 		},
 	},
 
@@ -127,7 +135,9 @@ export default {
 				// We have to do this synchronously, because in unload and beforeunload
 				// Promises, async and await are prohibited.
 				signalingKill()
-				leaveConversationSync(this.token)
+				if (!this.isLeavingAfterSessionConflict) {
+					leaveConversationSync(this.token)
+				}
 			}
 		})
 	},
diff --git a/src/components/TopBar/CallButton.vue b/src/components/TopBar/CallButton.vue
index 31608031e83..fb25c896c7a 100644
--- a/src/components/TopBar/CallButton.vue
+++ b/src/components/TopBar/CallButton.vue
@@ -51,6 +51,7 @@
 <script>
 import { CONVERSATION, PARTICIPANT, WEBINAR } from '../../constants'
 import browserCheck from '../../mixins/browserCheck'
+import isInCall from '../../mixins/isInCall'
 import Tooltip from '@nextcloud/vue/dist/Directives/Tooltip'
 import { emit } from '@nextcloud/event-bus'
 
@@ -61,7 +62,10 @@ export default {
 		Tooltip,
 	},
 
-	mixins: [browserCheck],
+	mixins: [
+		browserCheck,
+		isInCall,
+	],
 
 	data() {
 		return {
@@ -173,12 +177,12 @@ export default {
 
 		showStartCallButton() {
 			return this.conversation.readOnly === CONVERSATION.STATE.READ_WRITE
-				&& this.participant.inCall === PARTICIPANT.CALL_FLAG.DISCONNECTED
+				&& !this.isInCall
 		},
 
 		showLeaveCallButton() {
 			return this.conversation.readOnly === CONVERSATION.STATE.READ_WRITE
-				&& this.participant.inCall !== PARTICIPANT.CALL_FLAG.DISCONNECTED
+				&& this.isInCall
 		},
 	},
 
diff --git a/src/mixins/duplicateSessionHandler.js b/src/mixins/duplicateSessionHandler.js
new file mode 100644
index 00000000000..604eccb235b
--- /dev/null
+++ b/src/mixins/duplicateSessionHandler.js
@@ -0,0 +1,55 @@
+/**
+ * @copyright Copyright (c) 2019 Marco Ambrosini <marcoambrosini@pm.me>
+ *
+ * @author Marco Ambrosini <marcoambrosini@pm.me>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+import { generateUrl } from '@nextcloud/router'
+import { EventBus } from '../services/EventBus'
+import SessionStorage from '../services/SessionStorage'
+
+const talkHashCheck = {
+	data() {
+		return {
+			isLeavingAfterSessionConflict: false,
+		}
+	},
+
+	beforeDestroy() {
+		EventBus.$off('duplicateSessionDetected', this.duplicateSessionTriggered)
+	},
+
+	beforeMount() {
+		EventBus.$on('duplicateSessionDetected', this.duplicateSessionTriggered)
+	},
+
+	methods: {
+		duplicateSessionTriggered() {
+			this.isLeavingAfterSessionConflict = true
+			SessionStorage.removeItem('joined_conversation')
+			this.$nextTick(() => {
+				// Need to delay until next tick, otherwise the PreventUnload is still being triggered
+				// Putting the window in front with the warning and irritating the user
+				window.location = generateUrl('/apps/spreed/duplicate-session')
+			})
+		},
+	},
+}
+
+export default talkHashCheck
diff --git a/src/mixins/isInCall.js b/src/mixins/isInCall.js
new file mode 100644
index 00000000000..e6364771154
--- /dev/null
+++ b/src/mixins/isInCall.js
@@ -0,0 +1,60 @@
+/**
+ *
+ * @copyright Copyright (c) 2020 Joas Schilling <coding@schilljs.com>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+import { PARTICIPANT } from '../constants'
+import SessionStorage from '../services/SessionStorage'
+import { EventBus } from '../services/EventBus'
+
+/**
+ * A mixin to check whether the current session of a user is in a call or not.
+ *
+ * Components using this mixin require a "token" property and a "participant" property with, at least, the "inCall" property.
+ */
+export default {
+
+	data() {
+		return {
+			sessionStorageJoinedConversation: null,
+		}
+	},
+
+	computed: {
+		isInCall() {
+			return this.sessionStorageJoinedConversation === this.token
+				&& this.participant.inCall !== PARTICIPANT.CALL_FLAG.DISCONNECTED
+		},
+	},
+
+	beforeDestroy() {
+		EventBus.$off('joinedConversation', this.readSessionStorageJoinedConversation)
+	},
+
+	beforeMount() {
+		EventBus.$on('joinedConversation', this.readSessionStorageJoinedConversation)
+		this.sessionStorageJoinedConversation = SessionStorage.getItem('joined_conversation')
+	},
+
+	methods: {
+		readSessionStorageJoinedConversation() {
+			this.sessionStorageJoinedConversation = SessionStorage.getItem('joined_conversation')
+		},
+	},
+}
diff --git a/src/router/router.js b/src/router/router.js
index 9c144af18e4..8bf321154ca 100644
--- a/src/router/router.js
+++ b/src/router/router.js
@@ -25,6 +25,7 @@ import Router from 'vue-router'
 import { getRootUrl, generateUrl } from '@nextcloud/router'
 import MainView from '../views/MainView.vue'
 import NotFoundView from '../views/NotFoundView.vue'
+import SessionConflictView from '../views/SessionConflictView.vue'
 import WelcomeView from '../views/WelcomeView.vue'
 
 Vue.use(Router)
@@ -54,6 +55,12 @@ export default new Router({
 			component: NotFoundView,
 			props: true,
 		},
+		{
+			path: '/apps/spreed/duplicate-session',
+			name: 'duplicatesession',
+			component: SessionConflictView,
+			props: true,
+		},
 		{
 			path: '/call/:token',
 			name: 'conversation',
diff --git a/src/services/SessionStorage.js b/src/services/SessionStorage.js
new file mode 100644
index 00000000000..4f9c3f95930
--- /dev/null
+++ b/src/services/SessionStorage.js
@@ -0,0 +1,30 @@
+/**
+ * @copyright Copyright (c) 2020 Joas Schilling <coding@schilljs.com>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+import { getBuilder } from '@nextcloud/browser-storage'
+
+/**
+ * Note: This uses the browsers sessionStorage not the browserStorage.
+ * As per https://stackoverflow.com/q/20325763 this is NOT shared between tabs.
+ * For us this is the solution we were looking for, as it allows us to
+ * identify if a user reloaded a conversation in the same tab,
+ * or entered it in another tab.
+ */
+export default getBuilder('talk').clearOnLogout().build()
diff --git a/src/services/participantsService.js b/src/services/participantsService.js
index 1561cf69077..585a3e2f8fa 100644
--- a/src/services/participantsService.js
+++ b/src/services/participantsService.js
@@ -21,12 +21,19 @@
  */
 
 import axios from '@nextcloud/axios'
-import { generateOcsUrl } from '@nextcloud/router'
+import {
+	generateUrl,
+	generateOcsUrl,
+} from '@nextcloud/router'
+import { showError } from '@nextcloud/dialogs'
 import {
 	signalingJoinConversation,
 	signalingLeaveConversation,
 } from '../utils/webrtc/index'
 import { EventBus } from './EventBus'
+import SessionStorage from './SessionStorage'
+import { PARTICIPANT } from '../constants'
+import store from '../store/index'
 
 /**
  * Joins the current user to a conversation specified with
@@ -35,17 +42,88 @@ import { EventBus } from './EventBus'
  * @param {string} token The conversation token;
  */
 const joinConversation = async(token) => {
+	// When the token is in the last joined conversation, the user is reloading or force joining
+	const forceJoin = SessionStorage.getItem('joined_conversation') === token
+
 	try {
-		const response = await axios.post(generateOcsUrl('apps/spreed/api/v2', 2) + `room/${token}/participants/active`)
+		const response = await axios.post(generateOcsUrl('apps/spreed/api/v2', 2) + `room/${token}/participants/active`, {
+			force: forceJoin,
+		})
+
+		// Update the participant and actor session after a force join
+		store.dispatch('updateSessionId', {
+			token: token,
+			participantIdentifier: store.getters.getParticipantIdentifier(),
+			sessionId: response.data.ocs.data.sessionId,
+		})
+		store.dispatch('setCurrentParticipant', response.data.ocs.data)
+
 		// FIXME Signaling should not be synchronous
 		await signalingJoinConversation(token, response.data.ocs.data.sessionId)
+		SessionStorage.setItem('joined_conversation', token)
 		EventBus.$emit('joinedConversation')
 		return response
 	} catch (error) {
-		console.debug(error)
+		if (error.response.status === 409) {
+			const responseData = error.response.data.ocs.data
+			let maxLastPingAge = new Date().getTime() / 1000 - 40
+			if (responseData.inCall !== PARTICIPANT.CALL_FLAG.DISCONNECTED) {
+				// When the user is/was in a call, we accept 20 seconds more delay
+				maxLastPingAge -= 20
+			}
+			if (maxLastPingAge > responseData.lastPing) {
+				console.debug('Force joining automatically because the old session didn\'t ping for 40 seconds')
+				await forceJoinConversation(token)
+			} else {
+				await confirmForceJoinConversation(token)
+			}
+		} else {
+			console.debug(error)
+			showError(t('spreed', 'Failed to join the conversation. Try to reload the page.'))
+		}
 	}
 }
 
+const confirmForceJoinConversation = async(token) => {
+	// Little hack to check if the close button was used which we can't disable,
+	// not listen to when it was used.
+	const interval = setInterval(function() {
+		// eslint-disable-next-line no-undef
+		if ($('.oc-dialog-dim').length === 0) {
+			clearInterval(interval)
+			EventBus.$emit('duplicateSessionDetected')
+			window.location = generateUrl('/apps/spreed')
+		}
+	}, 3000)
+
+	await OC.dialogs.confirmDestructive(
+		t('spreed', 'You are trying to join a conversation while having an active session in another window or device. This is currently not supported by Nextcloud Talk. What do you want to do?'),
+		t('spreed', 'Duplicate session'),
+		{
+			type: OC.dialogs.YES_NO_BUTTONS,
+			confirm: t('spreed', 'Join here'),
+			confirmClasses: 'error',
+			cancel: t('spreed', 'Leave this page'),
+		},
+		decision => {
+			clearInterval(interval)
+			if (!decision) {
+				// Cancel
+				EventBus.$emit('duplicateSessionDetected')
+				window.location = generateUrl('/apps/spreed')
+			} else {
+				// Confirm
+				forceJoinConversation(token)
+			}
+		}
+	)
+}
+
+const forceJoinConversation = async(token) => {
+	SessionStorage.setItem('joined_conversation', token)
+	await joinConversation(token)
+}
+
 /**
  * Joins the current user to a conversation specified with
  * the token.
diff --git a/src/store/participantsStore.js b/src/store/participantsStore.js
index fc71d94130e..279cab6bf9e 100644
--- a/src/store/participantsStore.js
+++ b/src/store/participantsStore.js
@@ -205,6 +205,20 @@ const actions = {
 		commit('purgeParticipantsStore', token)
 	},
 
+	updateSessionId({ commit, getters }, { token, participantIdentifier, sessionId }) {
+		const index = getters.getParticipantIndex(token, participantIdentifier)
+		if (index === -1) {
+			console.error('Participant not found', participantIdentifier)
+			return
+		}
+
+		const updatedData = {
+			sessionId: sessionId,
+			inCall: PARTICIPANT.CALL_FLAG.DISCONNECTED,
+		}
+		commit('updateParticipant', { token, index, updatedData })
+	},
+
 	async joinCall({ commit, getters }, { token, participantIdentifier, flags }) {
 		const index = getters.getParticipantIndex(token, participantIdentifier)
 		if (index === -1) {
diff --git a/src/utils/signaling.js b/src/utils/signaling.js
index e3742879dcd..93c8e953b65 100644
--- a/src/utils/signaling.js
+++ b/src/utils/signaling.js
@@ -31,7 +31,10 @@ import { rejoinConversation } from '../services/participantsService'
 import CancelableRequest from './cancelableRequest'
 import { EventBus } from '../services/EventBus'
 import axios from '@nextcloud/axios'
-import { generateOcsUrl, generateUrl } from '@nextcloud/router'
+import {
+	generateOcsUrl,
+	generateUrl,
+} from '@nextcloud/router'
 import {
 	showError,
 	showWarning,
@@ -292,6 +295,7 @@ function Internal(settings) {
 	this.hideWarning = settings.hideWarning
 	this.spreedArrayConnection = []
 
+	this.pullMessageErrorToast = null
 	this.pullMessagesFails = 0
 	this.pullMessagesRequest = null
 
@@ -405,6 +409,11 @@ Signaling.Internal.prototype._startPullingMessages = function() {
 	request(token)
 		.then(function(result) {
 			this.pullMessagesFails = 0
+			if (this.pullMessageErrorToast) {
+				this.pullMessageErrorToast.hideToast()
+				this.pullMessageErrorToast = null
+			}
+
 			result.data.ocs.data.forEach(message => {
 				this._trigger('onBeforeReceiveMessage', [message])
 				switch (message.type) {
@@ -431,23 +440,39 @@ Signaling.Internal.prototype._startPullingMessages = function() {
 				// User navigated away in the meantime. Ignore
 			} else if (axios.isCancel(error)) {
 				console.debug('Pulling messages request was cancelled')
-			} else if (error.response && (error.response.status === 404 || error.response.status === 403)) {
-				console.error('Stop pulling messages because room does not exist or is not accessible')
+			} else if (error.response && error.response.status === 409) {
+				// Participant joined a second time and this session was killed
+				console.error('Session was killed but the conversation still exists')
 				this._trigger('pullMessagesStoppedOnFail')
-			} else if (token) {
-				if (this.pullMessagesFails >= 3) {
-					console.error('Stop pulling messages after repeated failures')
 
-					this._trigger('pullMessagesStoppedOnFail')
+				EventBus.$emit('duplicateSessionDetected')
+			} else if (error.response && (error.response.status === 404 || error.response.status === 403)) {
+				// Conversation was deleted or the user was removed
+				console.error('Conversation was not found anymore')
+				window.location = generateUrl('/apps/spreed/not-found')
+			} else if (token) {
+				if (this.pullMessagesFails === 1) {
+					this.pullMessageErrorToast = showError(t('spreed', 'Lost connection to signaling server. Trying to reconnect.'), {
+						timeout: -1,
+					})
+				}
+				if (this.pullMessagesFails === 30) {
+					if (this.pullMessageErrorToast) {
+						this.pullMessageErrorToast.hideToast()
+					}
 
+					// Giving up after 5 minutes
+					this.pullMessageErrorToast = showError(t('spreed', 'Lost connection to signaling server. Try to reload the page manually.'), {
+						timeout: -1,
+					})
 					return
 				}
 
 				this.pullMessagesFails++
-				// Retry to pull messages after 5 seconds
+				// Retry to pull messages after 10 seconds
 				window.setTimeout(function() {
 					this._startPullingMessages()
-				}.bind(this), 5000)
+				}.bind(this), 10000)
 			}
 		}.bind(this))
 }
@@ -494,6 +519,7 @@ function Standalone(settings, urls) {
 	this.initialReconnectIntervalMs = 1000
 	this.maxReconnectIntervalMs = 16000
 	this.reconnectIntervalMs = this.initialReconnectIntervalMs
+	this.ownSessionJoined = false
 	this.joinedUsers = {}
 	this.rooms = []
 	this.connect()
@@ -860,6 +886,8 @@ Signaling.Standalone.prototype.helloResponseReceived = function(data) {
 }
 
 Signaling.Standalone.prototype.joinRoom = function(token, sessionId) {
+	this.ownSessionJoined = false
+
 	if (!this.sessionId) {
 		if (this._pendingJoinRoomPromise && this._pendingJoinRoomPromise.token === token) {
 			return this._pendingJoinRoomPromise
@@ -1040,6 +1068,17 @@ Signaling.Standalone.prototype.processRoomEvent = function(data) {
 			for (i = 0; i < joinedUsers.length; i++) {
 				this.joinedUsers[joinedUsers[i].sessionid] = true
 				delete leftUsers[joinedUsers[i].sessionid]
+
+				if (this.settings.userId && joinedUsers[i].userid === this.settings.userId) {
+					if (this.ownSessionJoined && joinedUsers[i].sessionid !== this.sessionId) {
+						console.error('Duplicated session detected for the same user.')
+						EventBus.$emit('duplicateSessionDetected')
+					} else if (joinedUsers[i].sessionid === this.sessionId) {
+						// We are ignoring joins before we found our own message,
+						// as otherwise you get the warning for your own old session immediately
+						this.ownSessionJoined = true
+					}
+				}
 			}
 			leftUsers = Object.keys(leftUsers)
 			if (leftUsers.length) {
@@ -1081,8 +1120,19 @@ Signaling.Standalone.prototype.processRoomMessageEvent = function(data) {
 }
 
 Signaling.Standalone.prototype.processRoomListEvent = function(data) {
-	console.debug('Room list event', data)
-	EventBus.$emit('shouldRefreshConversations')
+	switch (data.event.type) {
+	case 'disinvite':
+		if (data.event.disinvite.roomid === this.currentRoomToken) {
+			console.error('User or session was removed from the conversation, redirecting')
+			EventBus.$emit('duplicateSessionDetected')
+			break
+		}
+		// eslint-disable-next-line no-fallthrough
+	default:
+		console.debug('Room list event', data)
+		EventBus.$emit('shouldRefreshConversations')
+		break
+	}
 }
 
 Signaling.Standalone.prototype.processRoomParticipantsEvent = function(data) {
diff --git a/src/views/MainView.vue b/src/views/MainView.vue
index 34d870e27eb..b7652bf777b 100644
--- a/src/views/MainView.vue
+++ b/src/views/MainView.vue
@@ -22,6 +22,7 @@ import LobbyScreen from '../components/LobbyScreen'
 import TopBar from '../components/TopBar/TopBar'
 import { PARTICIPANT } from '../constants'
 import isInLobby from '../mixins/isInLobby'
+import isInCall from '../mixins/isInCall'
 
 export default {
 	name: 'MainView',
@@ -34,7 +35,9 @@ export default {
 
 	mixins: [
 		isInLobby,
+		isInCall,
 	],
+
 	props: {
 		token: {
 			type: String,
@@ -65,7 +68,7 @@ export default {
 		},
 
 		showChatInSidebar() {
-			return this.participant.inCall !== PARTICIPANT.CALL_FLAG.DISCONNECTED
+			return this.isInCall
 		},
 	},
 
diff --git a/src/views/SessionConflictView.vue b/src/views/SessionConflictView.vue
new file mode 100644
index 00000000000..4b597460825
--- /dev/null
+++ b/src/views/SessionConflictView.vue
@@ -0,0 +1,15 @@
+<template>
+	<div id="emptycontent">
+		<div id="emptycontent-icon" class="icon-info" />
+		<h2>{{ t('spreed', 'Duplicate session') }}</h2>
+		<p class="emptycontent-additional">
+			{{ t('spreed','You joined the conversation in another window or device. This is currently not supported by Nextcloud Talk so this session was closed.') }}
+		</p>
+	</div>
+</template>
+
+<script>
+export default {
+	name: 'SessionConflictView',
+}
+</script>