### 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)

Vector<Plane> frustum;
	Vector3 cam_pos = _get_camera_position();
	for (int i = 0; i < 4; i++) {
		Vector3 a = _get_screen_to_space(box[i]);
		Vector3 b = _get_screen_to_space(box[(i + 1) % 4]);
		if (orthogonal) {
			frustum.push_back(Plane((a - b).normalized(), a));
		} else {
			frustum.push_back(Plane(a, b, cam_pos));
		}
	}
	Plane near(-_get_camera_normal(), cam_pos);
	near.d -= get_znear();
	frustum.push_back(near);
	Plane far = -near;
	far.d += get_zfar();
	frustum.push_back(far);
	if (spatial_editor->get_single_selected_node()) {
		Node3D *single_selected = spatial_editor->get_single_selected_node();
		Node3DEditorSelectedItem *se = editor_selection->get_node_editor_data<Node3DEditorSelectedItem>(single_selected);
		if (se) {
			Ref<EditorNode3DGizmo> old_gizmo;
			if (!clicked_wants_append) {
				se->subgizmos.clear();
				old_gizmo = se->gizmo;
				se->gizmo.unref();
			}
			bool found_subgizmos = false;
			Vector<Ref<Node3DGizmo>> gizmos = single_selected->get_gizmos();
			for (int j = 0; j < gizmos.size(); j++) {