diff --git a/papaparse.js b/papaparse.js
index 60ee489b..aa5ebddf 100755
--- a/papaparse.js
+++ b/papaparse.js
@@ -1457,6 +1457,37 @@ License: MIT
 			if (!input)
 				return returnable();
 
+			// Rename headers if there are duplicates
+			if (config.header)
+			{
+				var firstLine = input.split(newline)[0];
+				var headers = firstLine.split(delim);
+				var separator = '_';
+				var headerMap = [];
+				var headerCount = {};
+				var duplicateHeaders = false;
+
+				for (var j in headers) {
+					var header = headers[j];
+					if (isFunction(config.transformHeader))
+						header = config.transformHeader(header, j);
+					var headerName = header;
+
+					var count = headerCount[header] || 0;
+					if (count > 0) {
+						duplicateHeaders = true;
+						headerName = header + separator + count;
+					}
+					headerCount[header] = count + 1;
+
+					headerMap.push(headerName);
+				}
+				if (duplicateHeaders) {
+					var editedInput = input.split(newline);
+					editedInput[0] = headerMap.join(delim);
+					input = editedInput.join(newline);
+				}
+			}
 			if (fastMode || (fastMode !== false && input.indexOf(quoteChar) === -1))
 			{
 				var rows = input.split(newline);
diff --git a/tests/test-cases.js b/tests/test-cases.js
index 75a20db4..843ba33c 100644
--- a/tests/test-cases.js
+++ b/tests/test-cases.js
@@ -585,7 +585,25 @@ var CORE_PARSER_TESTS = [
 			data: [['a', 'b', 'c'], ['']],
 			errors: []
 		}
-	}
+	},
+	{
+		description: "Simple duplicate header names",
+		input: 'A,A,A,A\n1,2,3,4',
+		config: { header: true },
+		expected: {
+			data: [['A', 'A_1', 'A_2', 'A_3'], ['1', '2', '3', '4']],
+			errors: []
+		}
+	},
+	{
+		description: "Duplicate header names with headerTransform",
+		input: 'A,A,A,A\n1,2,3,4',
+		config: { header: true, transformHeader: function(header) { return header.toLowerCase(); } },
+		expected: {
+			data: [['a', 'a_1', 'a_2', 'a_3'], ['1', '2', '3', '4']],
+			errors: []
+		}
+	},
 ];
 
 describe('Core Parser Tests', function() {