### 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 camera->unproject_position(p_point) * subviewport_container->get_stretch_shrink();
}
Vector3 Node3DEditorViewport::_get_ray_pos(const Vector2 &p_pos) const {
	return camera->project_ray_origin(p_pos / subviewport_container->get_stretch_shrink());
}
Vector3 Node3DEditorViewport::_get_camera_normal() const {
	return -_get_camera_transform().basis.get_column(2);
}
Vector3 Node3DEditorViewport::_get_ray(const Vector2 &p_pos) const {
	return camera->project_ray_normal(p_pos / subviewport_container->get_stretch_shrink());
}
void Node3DEditorViewport::_clear_selected() {
	_edit.gizmo = Ref<EditorNode3DGizmo>();
	_edit.gizmo_handle = -1;
	_edit.gizmo_handle_secondary = false;
	_edit.gizmo_initial_value = Variant();
	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()) {
		se->subgizmos.clear();
		se->gizmo->redraw();
		se->gizmo.unref();
		spatial_editor->update_transform_gizmo();
	} else {
		editor_selection->clear();
		Node3DEditor::get_singleton()->edit(nullptr);
	}
}
void Node3DEditorViewport::_select_clicked(bool p_allow_locked) {