fix reordering of render order (followup 78c60a2)

(cherry picked from commit df4a41f)

src/gui/layertree/qgslayertreemapcanvasbridge.cpp

@@ -82,25 +82,31 @@ void QgsLayerTreeMapCanvasBridge::setCustomLayerOrder( const QStringList& order
     return;
 
   // verify that the new order is correct
-  QStringList defOrder = defaultLayerOrder();
-  QStringList sortedNewOrder = order;
+  QStringList defOrder( defaultLayerOrder() );
+  QStringList newOrder( order );
+  QStringList sortedNewOrder( order );
   qSort( defOrder );
   qSort( sortedNewOrder );
 
   if ( defOrder.size() < sortedNewOrder.size() )
   {
-    // some bad layers in the list?
+    // might contain bad layers, but also duplicates
+    QSet<QString> ids( defOrder.toSet() );
+
     for ( int i = 0; i < sortedNewOrder.size(); i++ )
     {
-      if ( i >= defOrder.size() || defOrder[i] != sortedNewOrder[i] )
+      if ( !ids.contains( sortedNewOrder[i] ) )
+      {
+        newOrder.removeAll( sortedNewOrder[i] );
         sortedNewOrder.removeAt( i-- );
+      }
     }
   }
 
   if ( defOrder != sortedNewOrder )
     return; // must be permutation of the default order
 
-  mCustomLayerOrder = order;
+  mCustomLayerOrder = newOrder;
   emit customLayerOrderChanged( mCustomLayerOrder );
 
   if ( mHasCustomLayerOrder )