### 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)

}
		switch (nav_mode) {
			case NAVIGATION_PAN: {
				_nav_pan(m, _get_warped_mouse_motion(m));
			} break;
			case NAVIGATION_ZOOM: {
				_nav_zoom(m, m->get_relative());
			} break;
			case NAVIGATION_ORBIT: {
				_nav_orbit(m, _get_warped_mouse_motion(m));
			} break;
			case NAVIGATION_LOOK: {
				_nav_look(m, _get_warped_mouse_motion(m));
			} break;
			default: {
			}
		}
	}
	Ref<InputEventMagnifyGesture> magnify_gesture = p_event;
	if (magnify_gesture.is_valid()) {
		if (is_freelook_active()) {
			scale_freelook_speed(magnify_gesture->get_factor());
		} else {
			scale_cursor_distance(1.0 / magnify_gesture->get_factor());
		}
	}
	Ref<InputEventPanGesture> pan_gesture = p_event;
	if (pan_gesture.is_valid()) {
		NavigationScheme nav_scheme = (NavigationScheme)EDITOR_GET("editors/3d/navigation/navigation_scheme").operator int();
		NavigationMode nav_mode = NAVIGATION_NONE;