### 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 (selection_results_menu.size() <= p_result) {
		return;
	}
	clicked = selection_results_menu[p_result].item->get_instance_id();
	if (clicked.is_valid()) {
		_select_clicked(spatial_editor->get_tool_mode() == Node3DEditor::TOOL_MODE_SELECT);
	}
	selection_results_menu.clear();
}
void Node3DEditorViewport::_selection_menu_hide() {
	selection_results.clear();
	selection_menu->clear();
	selection_menu->reset_size();
}
void Node3DEditorViewport::set_can_preview(Camera3D *p_preview) {