### 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 VIEW_RIGHT: {
			cursor.x_rot = 0;
			cursor.y_rot = -Math_PI / 2.0;
			set_message(TTR("Right View."), 2);
			view_type = VIEW_TYPE_RIGHT;
			_set_auto_orthogonal();
			_update_name();
		} break;
		case VIEW_FRONT: {
			cursor.x_rot = 0;
			cursor.y_rot = 0;
			set_message(TTR("Front View."), 2);
			view_type = VIEW_TYPE_FRONT;
			_set_auto_orthogonal();
			_update_name();
		} break;
		case VIEW_REAR: {
			cursor.x_rot = 0;
			cursor.y_rot = Math_PI;
			set_message(TTR("Rear View."), 2);
			view_type = VIEW_TYPE_REAR;
			_set_auto_orthogonal();
			_update_name();
		} break;
		case VIEW_CENTER_TO_ORIGIN: {
			cursor.pos = Vector3(0, 0, 0);
		} break;
		case VIEW_CENTER_TO_SELECTION: {
			focus_selection();
		} break;