### 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)

Dictionary d;
	d["position"] = cursor.pos;
	d["x_rotation"] = cursor.x_rot;
	d["y_rotation"] = cursor.y_rot;
	d["distance"] = cursor.distance;
	d["use_environment"] = camera->get_environment().is_valid();
	d["orthogonal"] = camera->get_projection() == Camera3D::PROJECTION_ORTHOGONAL;
	d["view_type"] = view_type;
	d["auto_orthogonal"] = auto_orthogonal;
	d["auto_orthogonal_enabled"] = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(VIEW_AUTO_ORTHOGONAL));
	/
	int display_mode = VIEW_DISPLAY_NORMAL;
	for (int i = VIEW_DISPLAY_NORMAL; i < VIEW_DISPLAY_ADVANCED; i++) {
		if (view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(i))) {
			display_mode = i;
			break;
		}
	}
	for (int i = VIEW_DISPLAY_ADVANCED + 1; i < VIEW_DISPLAY_MAX; i++) {
		if (display_submenu->is_item_checked(display_submenu->get_item_index(i))) {
			display_mode = i;
			break;
		}
	}
	d["display_mode"] = display_mode;