### 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 (ED_IS_SHORTCUT("spatial_editor/focus_selection", p_event)) {
			_menu_option(VIEW_CENTER_TO_SELECTION);
		}
		if (ED_IS_SHORTCUT("spatial_editor/align_transform_with_view", p_event)) {
			_menu_option(VIEW_ALIGN_TRANSFORM_WITH_VIEW);
		}
		if (ED_IS_SHORTCUT("spatial_editor/align_rotation_with_view", p_event)) {
			_menu_option(VIEW_ALIGN_ROTATION_WITH_VIEW);
		}
		if (ED_IS_SHORTCUT("spatial_editor/insert_anim_key", p_event)) {
			if (!get_selected_count() || _edit.mode != TRANSFORM_NONE) {
				return;
			}
			if (!AnimationPlayerEditor::get_singleton()->get_track_editor()->has_keying()) {
				set_message(TTR("Keying is disabled (no key inserted)."));
				return;
			}
			List<Node *> &selection = editor_selection->get_selected_node_list();
			for (Node *E : selection) {
				Node3D *sp = Object::cast_to<Node3D>(E);
				if (!sp) {
					continue;
				}
				spatial_editor->emit_signal(SNAME("transform_key_request"), sp, "", sp->get_transform());
			}
			set_message(TTR("Animation Key Inserted."));
		}
		if (ED_IS_SHORTCUT("spatial_editor/cancel_transform", p_event) && _edit.mode != TRANSFORM_NONE) {
			cancel_transform();