### 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()) {
			if (ED_IS_SHORTCUT("spatial_editor/instant_translate", p_event)) {
				begin_transform(TRANSFORM_TRANSLATE, true);
			}
			if (ED_IS_SHORTCUT("spatial_editor/instant_rotate", p_event)) {
				begin_transform(TRANSFORM_ROTATE, true);
			}
			if (ED_IS_SHORTCUT("spatial_editor/instant_scale", p_event)) {
				begin_transform(TRANSFORM_SCALE, true);
			}
		}
		
		if (!orthogonal && ED_IS_SHORTCUT("spatial_editor/freelook_toggle", p_event)) {
			set_freelook_active(!is_freelook_active());
		} else if (k->get_keycode() == Key::ESCAPE) {
			set_freelook_active(false);
		}
		if (k->get_keycode() == Key::SPACE) {
			if (!k->is_pressed()) {
				emit_signal(SNAME("toggle_maximize_view"), this);
			}
		}
		if (ED_IS_SHORTCUT("spatial_editor/decrease_fov", p_event)) {
			scale_fov(-0.05);
		}
		if (ED_IS_SHORTCUT("spatial_editor/increase_fov", p_event)) {
			scale_fov(0.05);
		}
		if (ED_IS_SHORTCUT("spatial_editor/reset_fov", p_event)) {