### 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)

Transform3D camera_transform;
	camera_transform.translate_local(p_cursor.pos);
	camera_transform.basis.rotate(Vector3(1, 0, 0), -p_cursor.x_rot);
	camera_transform.basis.rotate(Vector3(0, 1, 0), -p_cursor.y_rot);
	if (orthogonal) {
		camera_transform.translate_local(0, 0, (get_zfar() - get_znear()) / 2.0);
	} else {
		camera_transform.translate_local(0, 0, p_cursor.distance);
	}
	return camera_transform;
}
int Node3DEditorViewport::get_selected_count() const {
	const HashMap<Node *, Object *> &selection = editor_selection->get_selection();
	int count = 0;
	for (const KeyValue<Node *, Object *> &E : selection) {
		Node3D *sp = Object::cast_to<Node3D>(E.key);
		if (!sp) {
			continue;
		}
		Node3DEditorSelectedItem *se = editor_selection->get_node_editor_data<Node3DEditorSelectedItem>(sp);
		if (!se) {
			continue;
		}
		count++;
	}
	return count;
}
void Node3DEditorViewport::cancel_transform() {