### 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 (!is_freelook_active()) {
		return;
	}
	const FreelookNavigationScheme navigation_scheme = (FreelookNavigationScheme)EDITOR_GET("editors/3d/freelook/freelook_navigation_scheme").operator int();
	Vector3 forward;
	if (navigation_scheme == FREELOOK_FULLY_AXIS_LOCKED) {
		
		forward = Vector3(0, 0, -1).rotated(Vector3(0, 1, 0), camera->get_rotation().y);
	} else {
		
		forward = camera->get_transform().basis.xform(Vector3(0, 0, -1));
	}
	const Vector3 right = camera->get_transform().basis.xform(Vector3(1, 0, 0));
	Vector3 up;
	if (navigation_scheme == FREELOOK_PARTIALLY_AXIS_LOCKED || navigation_scheme == FREELOOK_FULLY_AXIS_LOCKED) {
		
		up = Vector3(0, 1, 0);
	} else {
		
		up = camera->get_transform().basis.xform(Vector3(0, 1, 0));
	}
	Vector3 direction;
	
	
	Input *inp = Input::get_singleton();