### 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)

preview = p_preview;
	if (!preview_camera->is_pressed() && !previewing_cinema) {
		preview_camera->set_visible(p_preview);
	}
}
void Node3DEditorViewport::update_transform_gizmo_view() {
	if (!is_visible_in_tree()) {
		return;
	}
	Transform3D xform = spatial_editor->get_gizmo_transform();
	Transform3D camera_xform = camera->get_transform();
	if (xform.origin.is_equal_approx(camera_xform.origin)) {
		for (int i = 0; i < 3; i++) {
			RenderingServer::get_singleton()->instance_set_visible(move_gizmo_instance[i], false);
			RenderingServer::get_singleton()->instance_set_visible(move_plane_gizmo_instance[i], false);
			RenderingServer::get_singleton()->instance_set_visible(rotate_gizmo_instance[i], false);
			RenderingServer::get_singleton()->instance_set_visible(scale_gizmo_instance[i], false);
			RenderingServer::get_singleton()->instance_set_visible(scale_plane_gizmo_instance[i], false);
			RenderingServer::get_singleton()->instance_set_visible(axis_gizmo_instance[i], false);
		}
		/
		RenderingServer::get_singleton()->instance_set_visible(rotate_gizmo_instance[3], false);
		return;
	}
	const Vector3 camz = -camera_xform.get_basis().get_column(2).normalized();
	const Vector3 camy = -camera_xform.get_basis().get_column(1).normalized();