### 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)

gizmos_menu->clear();
	for (int i = 0; i < gizmo_plugins_by_name.size(); ++i) {
		if (!gizmo_plugins_by_name[i]->can_be_hidden()) {
			continue;
		}
		String plugin_name = gizmo_plugins_by_name[i]->get_gizmo_name();
		const int plugin_state = gizmo_plugins_by_name[i]->get_state();
		gizmos_menu->add_multistate_item(plugin_name, 3, plugin_state, i);
		const int idx = gizmos_menu->get_item_index(i);
		gizmos_menu->set_item_tooltip(
				idx,
				TTR("Click to toggle between visibility states.\n\nOpen eye: Gizmo is visible.\nClosed eye: Gizmo is hidden.\nHalf-open eye: Gizmo is also visible through opaque surfaces (\"x-ray\")."));
		switch (plugin_state) {
			case EditorNode3DGizmoPlugin::VISIBLE:
				gizmos_menu->set_item_icon(idx, gizmos_menu->get_theme_icon(SNAME("visibility_visible")));
				break;
			case EditorNode3DGizmoPlugin::ON_TOP:
				gizmos_menu->set_item_icon(idx, gizmos_menu->get_theme_icon(SNAME("visibility_xray")));
				break;
			case EditorNode3DGizmoPlugin::HIDDEN:
				gizmos_menu->set_item_icon(idx, gizmos_menu->get_theme_icon(SNAME("visibility_hidden")));
				break;
		}
	}
}
void Node3DEditor::_update_gizmos_menu_theme() {