I'll explain the gag, since the clip is in German. The girl in the restroom is using the mirror to put on her makeup. But the mirror is really just a sheet of plain glass, and her "reflection" is actually her twin sister standing across the glass in a specially-built "mirror-image" room. (via The Presurfer)
Rufus T. Firefly and his doppelganger(s) in the famous "mirror scene" from Duck Soup.