Fix duplicate layers in custom layer order panel

Fixes #21955

src/core/layertree/qgslayertree.cpp

@@ -209,7 +209,18 @@ void QgsLayerTree::nodeRemovedChildren()
       ++layer;
   }
 
+  // we need to ensure that the customLayerOrderChanged signal is ALWAYS raised
+  // here, since that order HAS changed due to removal of the child!
+  // setCustomLayerOrder will only emit this signal when the layers list
+  // at this stage is different to the stored customer layer order. If this
+  // isn't the case (i.e. the lists ARE the same) then manually emit the
+  // signal
+  const bool emitSignal = _qgis_listRawToQPointer( layers ) == mCustomLayerOrder;
+
   setCustomLayerOrder( layers );
+  if ( emitSignal )
+    emit customLayerOrderChanged();
+
   emit layerOrderChanged();
 }
 

tests/src/python/test_qgslayertree.py

@@ -19,7 +19,8 @@
 from qgis.core import (
     QgsLayerTree,
     QgsProject,
-    QgsVectorLayer
+    QgsVectorLayer,
+    QgsLayerTreeLayer
 )
 from qgis.testing import start_app, unittest
 from utilities import (unitTestDataPath)
@@ -78,6 +79,28 @@ def testCustomLayerOrder(self):
         prj.clear()
         self.assertEqual(prj.layerTreeRoot().customLayerOrder(), [])
 
+    def testCustomLayerOrderChanged(self):
+        layer = QgsVectorLayer("Point?field=fldtxt:string",
+                               "layer1", "memory")
+        layer2 = QgsVectorLayer("Point?field=fldtxt:string",
+                                "layer2", "memory")
+
+        layer_tree = QgsLayerTree()
+        layer_order_changed_spy = QSignalSpy(layer_tree.customLayerOrderChanged)
+        layer1_node = QgsLayerTreeLayer(layer)
+        layer_tree.addChildNode(layer1_node)
+        self.assertEqual(len(layer_order_changed_spy), 1)
+        layer2_node = QgsLayerTreeLayer(layer2)
+        layer_tree.addChildNode(layer2_node)
+        self.assertEqual(len(layer_order_changed_spy), 2)
+
+        # simulate a layer move in the tree
+        layer3_node = QgsLayerTreeLayer(layer)
+        layer_tree.addChildNode(layer3_node)
+        self.assertEqual(len(layer_order_changed_spy), 3)
+        layer_tree.removeChildNode(layer1_node)
+        self.assertEqual(len(layer_order_changed_spy), 4)
+
 
 if __name__ == '__main__':
     unittest.main()