fix: keep marker keys unique when repeated across worlds

lib/src/layer/marker_layer/marker_layer.dart

@@ -138,7 +138,13 @@ class _MarkerLayerState extends State<MarkerLayer> {
                   Offset(shiftedX, pxPoint.dy) - pixelOrigin;
 
               return Positioned(
-                key: m.key,
+                // Keep the caller's key on the main world so find.byKey and
+                // per-marker identity are unchanged; derive a distinct key for
+                // each repeated world so a keyed marker does not produce
+                // duplicate sibling keys in the Stack (#2229).
+                key: m.key == null || worldShift == 0
+                    ? m.key
+                    : ValueKey((m.key, worldShift)),
                 width: m.width,
                 height: m.height,
                 left: shiftedLocalPoint.dx - right,

test/layer/marker_key_worlds_test.dart

@@ -0,0 +1,63 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_map/flutter_map.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:latlong2/latlong.dart';
+
+import '../test_utils/test_tile_provider.dart';
+
+void main() {
+  testWidgets(
+    'a keyed marker repeated across worlds does not produce duplicate keys (#2229)',
+    (tester) async {
+      const key = Key('m-1');
+      final markers = <Marker>[
+        const Marker(
+          key: key,
+          width: 20,
+          height: 20,
+          point: LatLng(0, 0),
+          child: FlutterLogo(),
+        ),
+      ];
+
+      // A wide viewport at low zoom shows the world several times, so a keyed
+      // marker is drawn once per visible world. Before the fix, those copies
+      // shared m.key and the layer's Stack threw "Duplicate keys".
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: Center(
+              child: SizedBox(
+                width: 900,
+                height: 200,
+                child: FlutterMap(
+                  options: const MapOptions(
+                    initialCenter: LatLng(0, 0),
+                    initialZoom: 0,
+                  ),
+                  children: [
+                    TileLayer(
+                      urlTemplate:
+                          'https://tile.openstreetmap.org/{z}/{x}/{y}.png',
+                      tileProvider: TestTileProvider(),
+                    ),
+                    MarkerLayer(markers: markers),
+                  ],
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+
+      expect(tester.takeException(), isNull);
+      // The marker really is drawn more than once (worlds repeated).
+      expect(
+        tester.widgetList(find.byType(FlutterLogo)).length,
+        greaterThan(1),
+      );
+      // The main world keeps the caller's key, so find.byKey resolves to one.
+      expect(find.byKey(key), findsOneWidget);
+    },
+  );
+}