### 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)

spatial_editor->edit(Object::cast_to<Node3D>(p_object));
}
bool Node3DEditorPlugin::handles(Object *p_object) const {
	return p_object->is_class("Node3D");
}
Dictionary Node3DEditorPlugin::get_state() const {
	return spatial_editor->get_state();
}
void Node3DEditorPlugin::set_state(const Dictionary &p_state) {
	spatial_editor->set_state(p_state);
}
Vector3 Node3DEditor::snap_point(Vector3 p_target, Vector3 p_start) const {
	if (is_snap_enabled()) {
		real_t snap = get_translate_snap();
		p_target.snap(Vector3(snap, snap, snap));
	}
	return p_target;
}
bool Node3DEditor::is_gizmo_visible() const {
	if (selected) {
		return gizmo.visible && selected->is_transform_gizmo_visible();
	}
	return gizmo.visible;
}
real_t Node3DEditor::get_translate_snap() const {