### Godot version

Master (4.2)

### System information

Windows 11 - Vulkan - Nvidia RTX 4070 - intel i5 13600KF

### Issue description

As you can see in the video when you try to add multiple scenes in to the 3D viewport (only with multiple scenes) the editor throws this error.

```
ERROR: Can't add child '@Node3D@18837' to '@SubViewport@9274', already has a parent '@SubViewport@9274'.
   at: (scene\main\node.cpp:1411)
```


Video: https://youtu.be/BVcCIigQJDw

This happens in the latest master (pulled on 2.11.2023).

### Steps to reproduce

Drag couple of the 3d mesh scenes into the 3D viewport and then select multiple scenes and drag them over and observer the error.

### Minimal reproduction project

[TestSceneDragAndDrop.zip](https://github.com/godotengine/godot/files/13253741/TestSceneDragAndDrop.zip)

break;
					}
					if (after != EditorPlugin::AFTER_GUI_INPUT_CUSTOM) {
						selection_in_progress = false;
						if (clicked.is_valid()) {
							_select_clicked(false);
						}
						if (cursor.region_select) {
							_select_region();
							cursor.region_select = false;
							surface->queue_redraw();
						}
					}
					if (_edit.mode != TRANSFORM_NONE) {
						Node3D *selected = spatial_editor->get_single_selected_node();
						Node3DEditorSelectedItem *se = selected ? editor_selection->get_node_editor_data<Node3DEditorSelectedItem>(selected) : nullptr;
						if (se && se->gizmo.is_valid()) {
							Vector<int> ids;
							Vector<Transform3D> restore;
							for (const KeyValue<int, Transform3D> &GE : se->subgizmos) {
								ids.push_back(GE.key);
								restore.push_back(GE.value);
							}
							se->gizmo->commit_subgizmos(ids, restore, false);
						} else {
							commit_transform();
						}
						_edit.mode = TRANSFORM_NONE;
						set_message("");
						spatial_editor->update_transform_gizmo();