### 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)

case NOTIFICATION_PROCESS: {
			real_t delta = get_process_delta_time();
			if (zoom_indicator_delay > 0) {
				zoom_indicator_delay -= delta;
				if (zoom_indicator_delay <= 0) {
					surface->queue_redraw();
					zoom_limit_label->hide();
				}
			}
			_update_navigation_controls_visibility();
			_update_freelook(delta);
			Node *scene_root = SceneTreeDock::get_singleton()->get_editor_data()->get_edited_scene_root();
			if (previewing_cinema && scene_root != nullptr) {
				Camera3D *cam = scene_root->get_viewport()->get_camera_3d();
				if (cam != nullptr && cam != previewing) {
					
					if (previewing != nullptr) {
						previewing->disconnect("tree_exited", callable_mp(this, &Node3DEditorViewport::_preview_exited_scene));
					}
					previewing = cam;
					previewing->connect("tree_exited", callable_mp(this, &Node3DEditorViewport::_preview_exited_scene));
					RS::get_singleton()->viewport_attach_camera(viewport->get_viewport_rid(), cam->get_camera());
					surface->queue_redraw();
				}
			}
			_update_camera(delta);
			const HashMap<Node *, Object *> &selection = editor_selection->get_selection();
			bool changed = false;
			bool exist = false;
			for (const KeyValue<Node *, Object *> &E : selection) {