### 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)

}
	}
	if (p_state.has("half_res")) {
		bool half_res = p_state["half_res"];
		int idx = view_menu->get_popup()->get_item_index(VIEW_HALF_RESOLUTION);
		view_menu->get_popup()->set_item_checked(idx, half_res);
		_update_shrink();
	}
	if (p_state.has("cinematic_preview")) {
		previewing_cinema = p_state["cinematic_preview"];
		int idx = view_menu->get_popup()->get_item_index(VIEW_CINEMATIC_PREVIEW);
		view_menu->get_popup()->set_item_checked(idx, previewing_cinema);
	}
	if (preview_camera->is_connected("toggled", callable_mp(this, &Node3DEditorViewport::_toggle_camera_preview))) {
		preview_camera->disconnect("toggled", callable_mp(this, &Node3DEditorViewport::_toggle_camera_preview));
	}
	if (p_state.has("previewing")) {
		Node *pv = EditorNode::get_singleton()->get_edited_scene()->get_node(p_state["previewing"]);
		if (Object::cast_to<Camera3D>(pv)) {
			previewing = Object::cast_to<Camera3D>(pv);
			previewing->connect("tree_exiting", callable_mp(this, &Node3DEditorViewport::_preview_exited_scene));
			RS::get_singleton()->viewport_attach_camera(viewport->get_viewport_rid(), previewing->get_camera()); /
			surface->queue_redraw();
			preview_camera->set_pressed(true);
			preview_camera->show();
		}
	}
	preview_camera->connect("toggled", callable_mp(this, &Node3DEditorViewport::_toggle_camera_preview));
}
Dictionary Node3DEditorViewport::get_state() const {