### 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: {
			ProjectSettings::get_singleton()->connect("settings_changed", callable_mp(this, &Node3DEditorViewport::_project_settings_changed));
		} break;
		case NOTIFICATION_VISIBILITY_CHANGED: {
			bool vp_visible = is_visible_in_tree();
			set_process(vp_visible);
			set_physics_process(vp_visible);
			if (vp_visible) {
				orthogonal = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(VIEW_ORTHOGONAL));
				_update_name();
				_update_camera(0);
			} else {
				set_freelook_active(false);
			}
			call_deferred(SNAME("update_transform_gizmo_view"));
		} break;
		case NOTIFICATION_RESIZED: {
			call_deferred(SNAME("update_transform_gizmo_view"));
		} break;