### 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 (p_what) {
		case NOTIFICATION_ENTER_TREE: {
			if (!is_connected("mouse_exited", callable_mp(this, &ViewportNavigationControl::_on_mouse_exited))) {
				connect("mouse_exited", callable_mp(this, &ViewportNavigationControl::_on_mouse_exited));
			}
			if (!is_connected("mouse_entered", callable_mp(this, &ViewportNavigationControl::_on_mouse_entered))) {
				connect("mouse_entered", callable_mp(this, &ViewportNavigationControl::_on_mouse_entered));
			}
		} break;
		case NOTIFICATION_DRAW: {
			if (viewport != nullptr) {
				_draw();
				_update_navigation();
			}
		} break;
	}
}
void ViewportNavigationControl::_draw() {