### 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)

if ((!seg.is_valid()) || found_gizmos.has(seg)) {
				continue;
			}
			found_gizmos.insert(seg);
			Vector3 point;
			Vector3 normal;
			bool inters = seg->intersect_ray(camera, shrinked_pos, point, normal);
			if (!inters) {
				continue;
			}
			const real_t dist = pos.distance_to(point);
			if (dist < 0) {
				continue;
			}
			if (dist < closest_dist) {
				item = Object::cast_to<Node>(spat);
				if (item != edited_scene) {
					item = edited_scene->get_deepest_editable_node(item);
				}
				closest = item->get_instance_id();
				closest_dist = dist;
			}
		}
	}
	if (!item) {
		return ObjectID();
	}
	return closest;
}
void Node3DEditorViewport::_find_items_at_pos(const Point2 &p_pos, Vector<_RayResult> &r_results, bool p_include_locked_nodes) {