### 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)

view_menu->get_popup()->add_check_shortcut(ED_SHORTCUT("spatial_editor/view_fps", TTR("View Frame Time")), VIEW_FRAME_TIME);
	view_menu->get_popup()->set_item_checked(view_menu->get_popup()->get_item_index(VIEW_ENVIRONMENT), true);
	view_menu->get_popup()->add_separator();
	view_menu->get_popup()->add_check_shortcut(ED_SHORTCUT("spatial_editor/view_half_resolution", TTR("Half Resolution")), VIEW_HALF_RESOLUTION);
	view_menu->get_popup()->add_separator();
	view_menu->get_popup()->add_check_shortcut(ED_SHORTCUT("spatial_editor/view_audio_listener", TTR("Audio Listener")), VIEW_AUDIO_LISTENER);
	view_menu->get_popup()->add_check_shortcut(ED_SHORTCUT("spatial_editor/view_audio_doppler", TTR("Enable Doppler")), VIEW_AUDIO_DOPPLER);
	view_menu->get_popup()->set_item_checked(view_menu->get_popup()->get_item_index(VIEW_GIZMOS), true);
	view_menu->get_popup()->add_separator();
	view_menu->get_popup()->add_check_shortcut(ED_SHORTCUT("spatial_editor/view_cinematic_preview", TTR("Cinematic Preview")), VIEW_CINEMATIC_PREVIEW);
	view_menu->get_popup()->add_separator();
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/focus_origin"), VIEW_CENTER_TO_ORIGIN);
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/focus_selection"), VIEW_CENTER_TO_SELECTION);
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/align_transform_with_view"), VIEW_ALIGN_TRANSFORM_WITH_VIEW);
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/align_rotation_with_view"), VIEW_ALIGN_ROTATION_WITH_VIEW);
	view_menu->get_popup()->connect("id_pressed", callable_mp(this, &Node3DEditorViewport::_menu_option));
	display_submenu->connect("id_pressed", callable_mp(this, &Node3DEditorViewport::_menu_option));
	view_menu->set_disable_shortcuts(true);
	/
	/
	if (false) {
		/
		const int normal_idx = view_menu->get_popup()->get_item_index(VIEW_DISPLAY_NORMAL);
		const int wireframe_idx = view_menu->get_popup()->get_item_index(VIEW_DISPLAY_WIREFRAME);
		const int overdraw_idx = view_menu->get_popup()->get_item_index(VIEW_DISPLAY_OVERDRAW);
		const int shadeless_idx = view_menu->get_popup()->get_item_index(VIEW_DISPLAY_UNSHADED);
		const String unsupported_tooltip = TTR("Not available when using the OpenGL renderer.");
		view_menu->get_popup()->set_item_disabled(normal_idx, true);
		view_menu->get_popup()->set_item_tooltip(normal_idx, unsupported_tooltip);
		view_menu->get_popup()->set_item_disabled(wireframe_idx, true);