### 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)

switch (p_what) {
		case NOTIFICATION_READY: {
			_menu_item_pressed(MENU_VIEW_USE_1_VIEWPORT);
			_refresh_menu_icons();
			get_tree()->connect("node_removed", callable_mp(this, &Node3DEditor::_node_removed));
			get_tree()->connect("node_added", callable_mp(this, &Node3DEditor::_node_added));
			SceneTreeDock::get_singleton()->get_tree_editor()->connect("node_changed", callable_mp(this, &Node3DEditor::_refresh_menu_icons));
			editor_selection->connect("selection_changed", callable_mp(this, &Node3DEditor::_selection_changed));
			EditorRunBar::get_singleton()->connect("stop_pressed", callable_mp(this, &Node3DEditor::_update_camera_override_button).bind(false));
			EditorRunBar::get_singleton()->connect("play_pressed", callable_mp(this, &Node3DEditor::_update_camera_override_button).bind(true));
			_update_preview_environment();
			sun_state->set_custom_minimum_size(sun_vb->get_combined_minimum_size());
			environ_state->set_custom_minimum_size(environ_vb->get_combined_minimum_size());
			ProjectSettings::get_singleton()->connect("settings_changed", callable_mp(this, &Node3DEditor::update_all_gizmos).bind(Variant()));
		} break;
		case NOTIFICATION_ENTER_TREE: {
			_update_theme();
			_register_all_gizmos();
			_update_gizmos_menu();
			_init_indicators();
			update_all_gizmos();
		} break;