[He does know who it is before the mirror confirms it, and it makes Mettaton pause.]

Zacharie, hm? You're his mirror. Is there any particular reason you've come waltzing into my life? Or do you just want to poke fun at my position in my game?

[It's moments like these where he wishes he could see the other side of the mirror. He wonders if Mirror Zacharie looks like his real or if something is fundamentally different.]