### 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)

Ref<EditorNode3DGizmo> seg = gizmos[j];
				if (!seg.is_valid()) {
					continue;
				}
				if (se->gizmo.is_valid() && se->gizmo != seg) {
					continue;
				}
				Vector<int> subgizmos = seg->subgizmos_intersect_frustum(camera, frustum);
				if (!subgizmos.is_empty()) {
					se->gizmo = seg;
					for (int i = 0; i < subgizmos.size(); i++) {
						int subgizmo_id = subgizmos[i];
						if (!se->subgizmos.has(subgizmo_id)) {
							se->subgizmos.insert(subgizmo_id, se->gizmo->get_subgizmo_transform(subgizmo_id));
						}
					}
					found_subgizmos = true;
					break;
				}
			}
			if (!clicked_wants_append || found_subgizmos) {
				if (se->gizmo.is_valid()) {
					se->gizmo->redraw();
				}
				if (old_gizmo != se->gizmo && old_gizmo.is_valid()) {
					old_gizmo->redraw();
				}
				spatial_editor->update_transform_gizmo();
			}
			if (found_subgizmos) {