diff --git a/extensions/vscode-api-tests/src/languages.test.ts b/extensions/vscode-api-tests/src/languages.test.ts
index dc2d933e4c1..fe6200c00a3 100644
--- a/extensions/vscode-api-tests/src/languages.test.ts
+++ b/extensions/vscode-api-tests/src/languages.test.ts
@@ -79,6 +79,55 @@ suite('languages namespace tests', () => {
 		collection.dispose();
 	});
 
+	test('diagnostics collection, set with dupliclated tuples', function () {
+		let collection = languages.createDiagnosticCollection('test');
+		let uri = Uri.parse('sc:hightower');
+		collection.set([
+			[uri, [new Diagnostic(new Range(0, 0, 0, 1), 'message-1')]],
+			[Uri.parse('some:thing'), [new Diagnostic(new Range(0, 0, 1, 1), 'something')]],
+			[uri, [new Diagnostic(new Range(0, 0, 0, 1), 'message-2')]],
+		]);
+
+		let array = collection.get(uri);
+		assert.equal(array.length, 2);
+		let [first, second] = array;
+		assert.equal(first.message, 'message-1');
+		assert.equal(second.message, 'message-2');
+
+		// clear
+		collection.delete(uri);
+		assert.ok(!collection.has(uri));
+
+		// bad tuple clears 1/2
+		collection.set([
+			[uri, [new Diagnostic(new Range(0, 0, 0, 1), 'message-1')]],
+			[Uri.parse('some:thing'), [new Diagnostic(new Range(0, 0, 1, 1), 'something')]],
+			[uri, undefined]
+		]);
+		assert.ok(!collection.has(uri));
+
+		// clear
+		collection.delete(uri);
+		assert.ok(!collection.has(uri));
+
+		// bad tuple clears 2/2
+		collection.set([
+			[uri, [new Diagnostic(new Range(0, 0, 0, 1), 'message-1')]],
+			[Uri.parse('some:thing'), [new Diagnostic(new Range(0, 0, 1, 1), 'something')]],
+			[uri, undefined],
+			[uri, [new Diagnostic(new Range(0, 0, 0, 1), 'message-2')]],
+			[uri, [new Diagnostic(new Range(0, 0, 0, 1), 'message-3')]],
+		]);
+
+		array = collection.get(uri);
+		assert.equal(array.length, 2);
+		[first, second] = array;
+		assert.equal(first.message, 'message-2');
+		assert.equal(second.message, 'message-3');
+
+		collection.dispose();
+	});
+
 	test('diagnostics & CodeActionProvider', function (done) {
 
 		class D2 extends Diagnostic {
diff --git a/src/vs/workbench/api/node/extHostDiagnostics.ts b/src/vs/workbench/api/node/extHostDiagnostics.ts
index 278050aae9b..3c9ca692e22 100644
--- a/src/vs/workbench/api/node/extHostDiagnostics.ts
+++ b/src/vs/workbench/api/node/extHostDiagnostics.ts
@@ -72,8 +72,19 @@ export class DiagnosticCollection implements vscode.DiagnosticCollection {
 			toSync = [];
 			for (let entry of first) {
 				let [uri, diagnostics] = entry;
-				this._data[uri.toString()] = diagnostics;
 				toSync.push(uri);
+				if (!diagnostics) {
+					// [Uri, undefined] means clear this
+					delete this._data[uri.toString()];
+				} else {
+					// set or merge diagnostics
+					let existing = this._data[uri.toString()];
+					if (existing) {
+						existing.push(...diagnostics);
+					} else {
+						this._data[uri.toString()] = diagnostics;
+					}
+				}
 			}
 		}