# UNDF: UNDF-2026-000000949
--- a/src/tiled/mapdocument.cpp
+++ b/src/tiled/mapdocument.cpp
@@ -1225,6 +1225,9 @@ static QList<Layer *> sortLayers(const Map &map, const QList<Layer *> &layers)
     if (layers.size() < 2)
         return layers;

+    const QSet<Layer *> layerSet(layers.begin(), layers.end());
+
     QList<Layer *> sorted;
     sorted.reserve(layers.size());

     LayerIterator iterator(&map);
     while (Layer *layer = iterator.next()) {
-        if (layers.contains(layer))
+        if (layerSet.contains(layer))
             sorted.append(layer);
     }

@@ -1247,6 +1250,9 @@ static QList<MapObject *> sortObjects(const Map &map, const QList<MapObject *> &
     if (objects.size() < 2)
         return objects;

+    const QSet<MapObject *> objectSet(objects.begin(), objects.end());
+
     QList<MapObject *> sorted;
     sorted.reserve(objects.size());

     LayerIterator iterator(&map);
     while (Layer *layer = iterator.next()) {
         if (layer->layerType() != Layer::ObjectGroupType)
             continue;

         for (MapObject *mapObject : static_cast<ObjectGroup*>(layer)->objects()) {
-            if (objects.contains(mapObject))
+            if (objectSet.contains(mapObject))
                 sorted.append(mapObject);
         }
     }
@@ -686,7 +686,8 @@ void MapDocument::duplicateLayers(const QList<Layer *> &layers)
     // Duplicate layers in the right order (groups before their children)
+    const QSet<Layer *> layerSet(layers.begin(), layers.end());
     LayerIterator iterator(mMap.get());
     iterator.toBack();
     while (Layer *layer = iterator.previous())
-        if (layers.contains(layer))
+        if (layerSet.contains(layer))
             layersToDuplicate.append(layer);

@@ -808,7 +809,8 @@ void MapDocument::moveLayersUp(const QList<Layer *> &layers)
     // Move layers in the right order, and abort if one of the layers can't be
     // moved (iterating backwards because when moving layers up we need to
     // start moving the top-most layer first)
+    const QSet<Layer *> layerSet(layers.begin(), layers.end());
     LayerIterator iterator(mMap.get());
     iterator.toBack();
     while (Layer *layer = iterator.previous()) {
-        if (layers.contains(layer)) {
+        if (layerSet.contains(layer)) {
             if (!MoveLayer::canMoveUp(*layer))
                 return;

@@ -837,6 +839,7 @@ void MapDocument::moveLayersDown(const QList<Layer *> &layers)
     // Move layers in the right order, and abort if one of the layers can't be moved
+    const QSet<Layer *> layerSet(layers.begin(), layers.end());
     for (Layer *layer : mMap->allLayers()) {
-        if (layers.contains(layer)) {
+        if (layerSet.contains(layer)) {
             if (!MoveLayer::canMoveDown(*layer))
                 return;