### 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)

Vector3 center;
	int count = 0;
	const List<Node *> &selection = editor_selection->get_selected_node_list();
	for (Node *E : selection) {
		Node3D *sp = Object::cast_to<Node3D>(E);
		if (!sp) {
			continue;
		}
		Node3DEditorSelectedItem *se = editor_selection->get_node_editor_data<Node3DEditorSelectedItem>(sp);
		if (!se) {
			continue;
		}
		if (se->gizmo.is_valid()) {
			for (const KeyValue<int, Transform3D> &GE : se->subgizmos) {
				center += se->gizmo->get_subgizmo_transform(GE.key).origin;
				count++;
			}
		}
		center += sp->get_global_gizmo_transform().origin;
		count++;
	}
	if (count != 0) {
		center /= count;
	}
	cursor.pos = center;
}
void Node3DEditorViewport::assign_pending_data_pointers(Node3D *p_preview_node, AABB *p_preview_bounds, AcceptDialog *p_accept) {