### 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)

hovered = false;
	queue_redraw();
	if (focused_index != -1 && focused_index != p_index) {
		return;
	}
	if (p_pressed) {
		if (p_position.distance_to(get_size() / 2.0) < get_size().x / 2.0) {
			focused_pos = p_position;
			focused_index = p_index;
			queue_redraw();
		}
	} else {
		focused_index = -1;
		if (Input::get_singleton()->get_mouse_mode() == Input::MOUSE_MODE_CAPTURED) {
			Input::get_singleton()->set_mouse_mode(Input::MOUSE_MODE_VISIBLE);
			Input::get_singleton()->warp_mouse(focused_mouse_start);
		}
	}
}
void ViewportNavigationControl::_process_drag(int p_index, Vector2 p_position, Vector2 p_relative_position) {