### 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());
	if (!is_visible_in_tree()) {
		return;
	}
	snap_key_enabled = Input::get_singleton()->is_key_pressed(Key::CTRL);
}
void Node3DEditor::_sun_environ_settings_pressed() {
	Vector2 pos = sun_environ_settings->get_screen_position() + sun_environ_settings->get_size();
	sun_environ_popup->set_position(pos - Vector2(sun_environ_popup->get_contents_minimum_size().width / 2, 0));
	sun_environ_popup->reset_size();
	sun_environ_popup->popup();
}
void Node3DEditor::_add_sun_to_scene(bool p_already_added_environment) {