### 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)

clicked_wants_append = b->is_shift_pressed();
					if (_edit.mode != TRANSFORM_NONE && _edit.instant) {
						commit_transform();
						break; 
					}
					NavigationScheme nav_scheme = (NavigationScheme)EDITOR_GET("editors/3d/navigation/navigation_scheme").operator int();
					if ((nav_scheme == NAVIGATION_MAYA || nav_scheme == NAVIGATION_MODO) && b->is_alt_pressed()) {
						break;
					}
					if (spatial_editor->get_tool_mode() == Node3DEditor::TOOL_MODE_LIST_SELECT) {
						_list_select(b);
						break;
					}
					_edit.mouse_pos = b->get_position();
					_edit.original_mouse_pos = b->get_position();
					_edit.snap = spatial_editor->is_snap_enabled();
					_edit.mode = TRANSFORM_NONE;
					_edit.original = spatial_editor->get_gizmo_transform(); 
					bool can_select_gizmos = spatial_editor->get_single_selected_node();
					{
						int idx = view_menu->get_popup()->get_item_index(VIEW_GIZMOS);
						can_select_gizmos = can_select_gizmos && view_menu->get_popup()->is_item_checked(idx);
					}
					
					if (can_select_gizmos) {
						Vector<Ref<Node3DGizmo>> gizmos = spatial_editor->get_single_selected_node()->get_gizmos();
						bool intersected_handle = false;
						for (int i = 0; i < gizmos.size(); i++) {
							Ref<EditorNode3DGizmo> seg = gizmos[i];
							if ((!seg.is_valid())) {