diff --git a/lib/Service/NegativeSampleGenerator.php b/lib/Service/NegativeSampleGenerator.php
index 8286f90a..6081bd13 100644
--- a/lib/Service/NegativeSampleGenerator.php
+++ b/lib/Service/NegativeSampleGenerator.php
@@ -22,6 +22,10 @@
 use function str_split;
 
 class NegativeSampleGenerator {
+	/**
+	 * Get IP vectors exclusively used by one user.
+	 * Includes the user vector in second dimension of the returned array.
+	 */
 	private function getUniqueIPsPerUser(Dataset $positives): array {
 		$map = [];
 
@@ -35,7 +39,7 @@ private function getUniqueIPsPerUser(Dataset $positives): array {
 				$map[$ipVecStr] = [
 					$uidVecStr,
 				];
-			} else {
+			} elseif (!in_array($uidVecStr, $map[$ipVecStr])) {
 				$map[$ipVecStr][] = $uidVecStr;
 			}
 		}
diff --git a/tests/Unit/Service/NegativeSampleGeneratorTest.php b/tests/Unit/Service/NegativeSampleGeneratorTest.php
index 246f53e9..2b1cd611 100644
--- a/tests/Unit/Service/NegativeSampleGeneratorTest.php
+++ b/tests/Unit/Service/NegativeSampleGeneratorTest.php
@@ -128,6 +128,37 @@ public function testGenerateMultipleShuffledFromLimitedUnique(): void {
 		self::assertCount(5, $result);
 	}
 
+	/**
+	 * DataSet can consist of multiple unique entries only. If not handled correctly,
+	 * this will result in an array without any IP. This tests the
+	 * correct handling. See GitHub issue #860 for more.
+	 * @return void
+	 */
+	public function testGenerateMultipleShuffledFromUniquesOnly(): void {
+		$positives = new Unlabeled([
+			array_merge(self::decToBitArray(1, 16), self::decToBitArray(1, 32)),
+			array_merge(self::decToBitArray(1, 16), self::decToBitArray(1, 32)),
+			array_merge(self::decToBitArray(1, 16), self::decToBitArray(1, 32)),
+
+			array_merge(self::decToBitArray(2, 16), self::decToBitArray(2, 32)),
+			array_merge(self::decToBitArray(2, 16), self::decToBitArray(2, 32)),
+			array_merge(self::decToBitArray(2, 16), self::decToBitArray(2, 32)),
+		]);
+
+		$result = $this->generator->generateShuffledFromPositiveSamples($positives, 2);
+
+		self::assertCount(2, $result);
+		foreach ($result as $sample) {
+			$ipVec = array_slice($sample, 16, 32);
+
+			self::assertTrue(
+				$ipVec === self::decToBitArray(1, 32) ||
+				$ipVec === self::decToBitArray(2, 32),
+				'Sample has an unique IP'
+			);
+		}
+	}
+
 	/**
 	 * @return int[]
 	 */