### 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)

ERR_FAIL_COND(p_event.is_null());
	
	const Ref<InputEventMouseButton> mb = p_event;
	if (mb.is_valid() && mb->get_button_index() == MouseButton::LEFT) {
		_process_click(100, mb->get_position(), mb->is_pressed());
	}
	const Ref<InputEventMouseMotion> mm = p_event;
	if (mm.is_valid()) {
		_process_drag(mm, 100, mm->get_global_position(), viewport->_get_warped_mouse_motion(mm));
	}
	
	const Ref<InputEventScreenTouch> screen_touch = p_event;
	if (screen_touch.is_valid()) {
		_process_click(screen_touch->get_index(), screen_touch->get_position(), screen_touch->is_pressed());
	}
	const Ref<InputEventScreenDrag> screen_drag = p_event;
	if (screen_drag.is_valid()) {
		_process_drag(screen_drag, screen_drag->get_index(), screen_drag->get_position(), screen_drag->get_relative());
	}
}
void ViewportRotationControl::_update_focus() {