### 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)

d["listener"] = viewport->is_audio_listener_3d();
	d["doppler"] = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(VIEW_AUDIO_DOPPLER));
	d["gizmos"] = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(VIEW_GIZMOS));
	d["information"] = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(VIEW_INFORMATION));
	d["frame_time"] = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(VIEW_FRAME_TIME));
	d["half_res"] = subviewport_container->get_stretch_shrink() > 1;
	d["cinematic_preview"] = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(VIEW_CINEMATIC_PREVIEW));
	if (previewing) {
		d["previewing"] = EditorNode::get_singleton()->get_edited_scene()->get_path_to(previewing);
	}
	d["lock_rotation"] = lock_rotation;
	return d;
}
void Node3DEditorViewport::_bind_methods() {
	ClassDB::bind_method(D_METHOD("update_transform_gizmo_view"), &Node3DEditorViewport::update_transform_gizmo_view); /
	ADD_SIGNAL(MethodInfo("toggle_maximize_view", PropertyInfo(Variant::OBJECT, "viewport")));
	ADD_SIGNAL(MethodInfo("clicked", PropertyInfo(Variant::OBJECT, "viewport")));
}
void Node3DEditorViewport::reset() {
	orthogonal = false;
	auto_orthogonal = false;
	lock_rotation = false;
	message_time = 0;
	message = "";
	last_message = "";
	view_type = VIEW_TYPE_USER;
	cursor = Cursor();
	_update_name();
}
void Node3DEditorViewport::focus_selection() {