### 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)

return p_a->get_gizmo_name() < p_b->get_gizmo_name();
	}
};
void Node3DEditor::add_gizmo_plugin(Ref<EditorNode3DGizmoPlugin> p_plugin) {
	ERR_FAIL_NULL(p_plugin.ptr());
	gizmo_plugins_by_priority.push_back(p_plugin);
	gizmo_plugins_by_priority.sort_custom<_GizmoPluginPriorityComparator>();
	gizmo_plugins_by_name.push_back(p_plugin);
	gizmo_plugins_by_name.sort_custom<_GizmoPluginNameComparator>();
	_update_gizmos_menu();
}
void Node3DEditor::remove_gizmo_plugin(Ref<EditorNode3DGizmoPlugin> p_plugin) {
	gizmo_plugins_by_priority.erase(p_plugin);
	gizmo_plugins_by_name.erase(p_plugin);
	_update_gizmos_menu();
}
Node3DEditorPlugin::Node3DEditorPlugin() {
	spatial_editor = memnew(Node3DEditor);
	spatial_editor->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	EditorNode::get_singleton()->get_main_screen_control()->add_child(spatial_editor);
	spatial_editor->hide();
}
Node3DEditorPlugin::~Node3DEditorPlugin() {
}