saving a scene duplicates all lightmaps

I have a lightmaped scene, and I want to create a version of it with a different skybox.
But when just give the second a scene a different skybox and save the scene, Unreal duplicates all data include all lightmaps.

How can I avoid this? I just want to have a variant of this scene with a different skybox and I don’t want my project to duplicate unnecessary lighhtmaps.