### 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)

tool_option_button[TOOL_OPT_USE_SNAP]->connect("toggled", callable_mp(this, &Node3DEditor::_menu_item_toggled).bind(MENU_TOOL_USE_SNAP));
	tool_option_button[TOOL_OPT_USE_SNAP]->set_shortcut(ED_SHORTCUT("spatial_editor/snap", TTR("Use Snap"), Key::Y));
	tool_option_button[TOOL_OPT_USE_SNAP]->set_shortcut_context(this);
	main_menu_hbox->add_child(memnew(VSeparator));
	tool_option_button[TOOL_OPT_OVERRIDE_CAMERA] = memnew(Button);
	main_menu_hbox->add_child(tool_option_button[TOOL_OPT_OVERRIDE_CAMERA]);
	tool_option_button[TOOL_OPT_OVERRIDE_CAMERA]->set_toggle_mode(true);
	tool_option_button[TOOL_OPT_OVERRIDE_CAMERA]->set_theme_type_variation("FlatButton");
	tool_option_button[TOOL_OPT_OVERRIDE_CAMERA]->set_disabled(true);
	tool_option_button[TOOL_OPT_OVERRIDE_CAMERA]->connect("toggled", callable_mp(this, &Node3DEditor::_menu_item_toggled).bind(MENU_TOOL_OVERRIDE_CAMERA));
	_update_camera_override_button(false);
	main_menu_hbox->add_child(memnew(VSeparator));
	sun_button = memnew(Button);
	sun_button->set_tooltip_text(TTR("Toggle preview sunlight.\nIf a DirectionalLight3D node is added to the scene, preview sunlight is disabled."));
	sun_button->set_toggle_mode(true);
	sun_button->set_theme_type_variation("FlatButton");
	sun_button->connect("pressed", callable_mp(this, &Node3DEditor::_update_preview_environment), CONNECT_DEFERRED);
	/
	sun_button->set_pressed(true);
	main_menu_hbox->add_child(sun_button);
	environ_button = memnew(Button);
	environ_button->set_tooltip_text(TTR("Toggle preview environment.\nIf a WorldEnvironment node is added to the scene, preview environment is disabled."));
	environ_button->set_toggle_mode(true);
	environ_button->set_theme_type_variation("FlatButton");
	environ_button->connect("pressed", callable_mp(this, &Node3DEditor::_update_preview_environment), CONNECT_DEFERRED);
	/
	environ_button->set_pressed(true);
	main_menu_hbox->add_child(environ_button);
	sun_environ_settings = memnew(Button);
	sun_environ_settings->set_tooltip_text(TTR("Edit Sun and Environment settings."));