### 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)

bool listener = p_state["listener"];
		int idx = view_menu->get_popup()->get_item_index(VIEW_AUDIO_LISTENER);
		viewport->set_as_audio_listener_3d(listener);
		view_menu->get_popup()->set_item_checked(idx, listener);
	}
	if (p_state.has("doppler")) {
		bool doppler = p_state["doppler"];
		int idx = view_menu->get_popup()->get_item_index(VIEW_AUDIO_DOPPLER);
		camera->set_doppler_tracking(doppler ? Camera3D::DOPPLER_TRACKING_IDLE_STEP : Camera3D::DOPPLER_TRACKING_DISABLED);
		view_menu->get_popup()->set_item_checked(idx, doppler);
	}
	if (p_state.has("gizmos")) {
		bool gizmos = p_state["gizmos"];
		int idx = view_menu->get_popup()->get_item_index(VIEW_GIZMOS);
		if (view_menu->get_popup()->is_item_checked(idx) != gizmos) {
			_menu_option(VIEW_GIZMOS);
		}
	}
	if (p_state.has("information")) {
		bool information = p_state["information"];
		int idx = view_menu->get_popup()->get_item_index(VIEW_INFORMATION);
		if (view_menu->get_popup()->is_item_checked(idx) != information) {
			_menu_option(VIEW_INFORMATION);
		}
	}
	if (p_state.has("frame_time")) {
		bool fps = p_state["frame_time"];
		int idx = view_menu->get_popup()->get_item_index(VIEW_FRAME_TIME);
		if (view_menu->get_popup()->is_item_checked(idx) != fps) {
			_menu_option(VIEW_FRAME_TIME);