### 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())) {
								continue;
							}
							int subgizmo_id = seg->subgizmos_intersect_ray(camera, _edit.mouse_pos);
							if (subgizmo_id != -1) {
								ERR_CONTINUE(!se);
								if (b->is_shift_pressed()) {
									if (se->subgizmos.has(subgizmo_id)) {
										se->subgizmos.erase(subgizmo_id);
									} else {
										se->subgizmos.insert(subgizmo_id, seg->get_subgizmo_transform(subgizmo_id));
									}
								} else {
									se->subgizmos.clear();
									se->subgizmos.insert(subgizmo_id, seg->get_subgizmo_transform(subgizmo_id));
								}
								if (se->subgizmos.is_empty()) {
									se->gizmo = Ref<EditorNode3DGizmo>();
								} else {
									se->gizmo = seg;
								}
								seg->redraw();
								spatial_editor->update_transform_gizmo();
								intersected_subgizmo = true;
								break;
							}
						}
						if (intersected_subgizmo) {
							break;
						}