### 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)

const int idx = gizmos_menu->get_item_index(p_option);
	gizmos_menu->toggle_item_multistate(idx);
	/
	const int state = gizmos_menu->get_item_state(idx);
	switch (state) {
		case EditorNode3DGizmoPlugin::VISIBLE:
			gizmos_menu->set_item_icon(idx, view_menu->get_popup()->get_theme_icon(SNAME("visibility_visible")));
			break;
		case EditorNode3DGizmoPlugin::ON_TOP:
			gizmos_menu->set_item_icon(idx, view_menu->get_popup()->get_theme_icon(SNAME("visibility_xray")));
			break;
		case EditorNode3DGizmoPlugin::HIDDEN:
			gizmos_menu->set_item_icon(idx, view_menu->get_popup()->get_theme_icon(SNAME("visibility_hidden")));
			break;
	}
	gizmo_plugins_by_name.write[p_option]->set_state(state);
	update_all_gizmos();
}
void Node3DEditor::_update_camera_override_button(bool p_game_running) {
	Button *const button = tool_option_button[TOOL_OPT_OVERRIDE_CAMERA];
	if (p_game_running) {
		button->set_disabled(false);
		button->set_tooltip_text(TTR("Project Camera Override\nOverrides the running project's camera with the editor viewport camera."));
	} else {
		button->set_disabled(true);
		button->set_pressed(false);
		button->set_tooltip_text(TTR("Project Camera Override\nNo project instance running. Run the project from the editor to use this feature."));
	}
}
void Node3DEditor::_update_camera_override_viewport(Object *p_viewport) {