### 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)

bool shrink = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(VIEW_HALF_RESOLUTION));
	subviewport_container->set_stretch_shrink(shrink ? 2 : 1);
	subviewport_container->set_texture_filter(shrink ? TEXTURE_FILTER_NEAREST : TEXTURE_FILTER_PARENT_NODE);
}
float Node3DEditorViewport::get_znear() const {
	return CLAMP(spatial_editor->get_znear(), MIN_Z, MAX_Z);
}
float Node3DEditorViewport::get_zfar() const {
	return CLAMP(spatial_editor->get_zfar(), MIN_Z, MAX_Z);
}
float Node3DEditorViewport::get_fov() const {
	return CLAMP(spatial_editor->get_fov() * cursor.fov_scale, MIN_FOV, MAX_FOV);
}
Transform3D Node3DEditorViewport::_get_camera_transform() const {
	return camera->get_global_transform();
}
Vector3 Node3DEditorViewport::_get_camera_position() const {
	return _get_camera_transform().origin;
}
Point2 Node3DEditorViewport::_point_to_screen(const Vector3 &p_point) {