### 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)

d["show_grid"] = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_GRID));
	d["show_origin"] = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_ORIGIN));
	d["fov"] = get_fov();
	d["znear"] = get_znear();
	d["zfar"] = get_zfar();
	Dictionary gizmos_status;
	for (int i = 0; i < gizmo_plugins_by_name.size(); i++) {
		if (!gizmo_plugins_by_name[i]->can_be_hidden()) {
			continue;
		}
		int state = gizmos_menu->get_item_state(gizmos_menu->get_item_index(i));
		String name = gizmo_plugins_by_name[i]->get_gizmo_name();
		gizmos_status[name] = state;
	}
	d["gizmos_status"] = gizmos_status;
	{
		Dictionary pd;
		pd["sun_rotation"] = sun_rotation;
		pd["environ_sky_color"] = environ_sky_color->get_pick_color();
		pd["environ_ground_color"] = environ_ground_color->get_pick_color();
		pd["environ_energy"] = environ_energy->get_value();
		pd["environ_glow_enabled"] = environ_glow_button->is_pressed();
		pd["environ_tonemap_enabled"] = environ_tonemap_button->is_pressed();
		pd["environ_ao_enabled"] = environ_ao_button->is_pressed();
		pd["environ_gi_enabled"] = environ_gi_button->is_pressed();
		pd["sun_max_distance"] = sun_max_distance->get_value();
		pd["sun_color"] = sun_color->get_pick_color();
		pd["sun_energy"] = sun_energy->get_value();
		pd["sun_enabled"] = sun_button->is_pressed();
		pd["environ_enabled"] = environ_button->is_pressed();