### 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)

RenderingServer::get_singleton()->free(origin_instance);
	RenderingServer::get_singleton()->free(origin);
	_finish_grid();
}
void Node3DEditor::_finish_grid() {
	for (int i = 0; i < 3; i++) {
		RenderingServer::get_singleton()->free(grid_instance[i]);
		RenderingServer::get_singleton()->free(grid[i]);
	}
}
void Node3DEditor::update_grid() {
	const Camera3D::ProjectionType current_projection = viewports[0]->camera->get_projection();
	if (current_projection != grid_camera_last_update_perspective) {
		grid_init_draw = false; /
		grid_camera_last_update_perspective = current_projection;
	}
	/
	const Vector3 camera_position = get_editor_viewport(0)->camera->get_position();
	if (!grid_init_draw || grid_camera_last_update_position.distance_squared_to(camera_position) >= 100.0f) {
		_finish_grid();
		_init_grid();
		grid_init_draw = true;
		grid_camera_last_update_position = camera_position;
	}
}
void Node3DEditor::_selection_changed() {