### 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(!_edit.instant);
	Ref<InputEventMouseMotion> m = p_event;
	if (m.is_valid()) {
		if (_edit.mode == TRANSFORM_ROTATE) {
			_edit.mouse_pos = m->get_position(); 
		} else {
			_edit.mouse_pos += _get_warped_mouse_motion(p_event);
		}
		update_transform(_get_key_modifier(m) == Key::SHIFT);
	}
}
void Node3DEditorViewport::_sinput(const Ref<InputEvent> &p_event) {