4) "You're especially cute today."
4) It's her twin sister. (Yes, I've seen it happen.)
Fo' real?
In real life or in fiction?
Lol not in real life. I don't think it ever happened to me or any of my friends in real life... but it certainly can happen, especially if the twins make an effort to look identical.
In manga/games/anime, I've seen it happen many times. It happened to the MC in Higurashi no Naku Koro ni: he just couldn't tell apart the green-haired twins. In fact it took him a long time to realize they were two girls, not just one girl who suffered from curious mood swings. (And when he found out, it was in the most embarrassing way...)
Closer to home: in yuri manga, I remember it happening in the first pages of Narcissus Twins. Some loser guy claims to love one of the twins, but then mistakes her for her sister and ridicules himself. (We needn't feel sorry for him because he didn't have a chance to begin with, as those two were wincest sisters, haha.)