### 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)

if (inp->is_action_pressed("spatial_editor/freelook_left")) {
		direction -= right;
	}
	if (inp->is_action_pressed("spatial_editor/freelook_right")) {
		direction += right;
	}
	if (inp->is_action_pressed("spatial_editor/freelook_forward")) {
		direction += forward;
	}
	if (inp->is_action_pressed("spatial_editor/freelook_backwards")) {
		direction -= forward;
	}
	if (inp->is_action_pressed("spatial_editor/freelook_up")) {
		direction += up;
	}
	if (inp->is_action_pressed("spatial_editor/freelook_down")) {
		direction -= up;
	}
	real_t speed = freelook_speed;
	if (inp->is_action_pressed("spatial_editor/freelook_speed_modifier")) {
		speed *= 3.0;
	}
	if (inp->is_action_pressed("spatial_editor/freelook_slow_modifier")) {
		speed *= 0.333333;
	}
	const Vector3 motion = direction * speed * delta;
	cursor.pos += motion;
	cursor.eye_pos += motion;
}
void Node3DEditorViewport::set_message(String p_message, float p_time) {