Assert when duplicated keys are introduced in subsequent build (#81850)

packages/flutter/lib/src/widgets/framework.dart

@@ -6304,6 +6304,7 @@ class MultiChildRenderObjectElement extends RenderObjectElement {
   void update(MultiChildRenderObjectWidget newWidget) {
     super.update(newWidget);
     assert(widget == newWidget);
+    assert(!debugChildrenHaveDuplicateKeys(widget, widget.children));
     _children = updateChildren(_children, widget.children, forgottenChildren: _forgottenChildren);
     _forgottenChildren.clear();
   }

packages/flutter/test/widgets/framework_test.dart

@@ -512,11 +512,9 @@ void main() {
     expect(
       exception.toString(),
       equalsIgnoringHashCodes(
-        'Multiple widgets used the same GlobalKey.\n'
-        'The key [GlobalKey#00000 problematic] was used by 2 widgets:\n'
-        '  SizedBox-[GlobalKey#00000 problematic]\n'
-        '  Placeholder-[GlobalKey#00000 problematic]\n'
-        'A GlobalKey can only be specified on one widget at a time in the widget tree.',
+        'Duplicate keys found.\n'
+        'If multiple keyed nodes exist as children of another node, they must have unique keys.\n'
+        'Stack(alignment: AlignmentDirectional.topStart, textDirection: ltr, fit: loose) has multiple children with key [GlobalKey#00000 problematic].'
       ),
     );
   });
@@ -541,11 +539,9 @@ void main() {
     expect(
       exception.toString(),
       equalsIgnoringHashCodes(
-        'Multiple widgets used the same GlobalKey.\n'
-        'The key [GlobalKey#00000 problematic] was used by 2 widgets:\n'
-        '  Container-[GlobalKey#00000 problematic]\n'
-        '  Placeholder-[GlobalKey#00000 problematic]\n'
-        'A GlobalKey can only be specified on one widget at a time in the widget tree.',
+        'Duplicate keys found.\n'
+        'If multiple keyed nodes exist as children of another node, they must have unique keys.\n'
+        'Stack(alignment: AlignmentDirectional.topStart, textDirection: ltr, fit: loose) has multiple children with key [GlobalKey#00000 problematic].'
       ),
     );
   });

packages/flutter/test/widgets/multichildobject_with_keys_test.dart

@@ -58,6 +58,57 @@ void main() {
       <String>['0', '6', '7', '8', '1', '2', '3', '4', '5'],
     );
   });
+
+  testWidgets('Building a new MultiChildRenderObjectElement with children having duplicated keys throws', (WidgetTester tester) async {
+    const ValueKey<int> duplicatedKey = ValueKey<int>(1);
+
+    await tester.pumpWidget(Column(
+      children: const <Widget>[
+        Text('Text 1', textDirection: TextDirection.ltr, key: duplicatedKey),
+        Text('Text 2', textDirection: TextDirection.ltr, key: duplicatedKey),
+      ],
+    ));
+
+    expect(
+      tester.takeException(),
+      isA<FlutterError>().having(
+        (FlutterError error) => error.message,
+        'error.message',
+        startsWith('Duplicate keys found.'),
+      ),
+    );
+  });
+
+  testWidgets('Updating a MultiChildRenderObjectElement to have children with duplicated keys throws', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/81541
+
+    const ValueKey<int> key1 = ValueKey<int>(1);
+    const ValueKey<int> key2 = ValueKey<int>(2);
+
+    Future<void> _buildWithKey(Key key) {
+      return tester.pumpWidget(Column(
+        children: <Widget>[
+          const Text('Text 1', textDirection: TextDirection.ltr, key: key1),
+          Text('Text 2', textDirection: TextDirection.ltr, key: key),
+        ],
+      ));
+    }
+
+    // Initial build with two different keys.
+    await _buildWithKey(key2);
+    expect(tester.takeException(), isNull);
+
+    // Subsequent build with duplicated keys.
+    await _buildWithKey(key1);
+    expect(
+      tester.takeException(),
+      isA<FlutterError>().having(
+        (FlutterError error) => error.message,
+        'error.message',
+        startsWith('Duplicate keys found.'),
+      ),
+    );
+  });
 }
 
 // Do not use tester.renderObjectList(find.byType(RenderParagraph). That returns