### 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_activate && !preview);
	ERR_FAIL_COND(!p_activate && !previewing);
	previewing_camera = p_activate;
	_update_navigation_controls_visibility();
	if (!p_activate) {
		previewing->disconnect("tree_exiting", callable_mp(this, &Node3DEditorViewport::_preview_exited_scene));
		previewing = nullptr;
		RS::get_singleton()->viewport_attach_camera(viewport->get_viewport_rid(), camera->get_camera()); 
		if (!preview) {
			preview_camera->hide();
		}
		surface->queue_redraw();
	} else {
		previewing = preview;
		previewing->connect("tree_exiting", callable_mp(this, &Node3DEditorViewport::_preview_exited_scene));
		RS::get_singleton()->viewport_attach_camera(viewport->get_viewport_rid(), preview->get_camera()); /
		surface->queue_redraw();
	}
}
void Node3DEditorViewport::_toggle_cinema_preview(bool p_activate) {