Mary-Kate Olsen -- I Used to Be a Twin

Ashley Olsen (right) attended the 2014 World Of Children Awards in New York City on Thursday night alongside her formerly twin sister Mary-Kate.

28-year-old MK ... who's engaged to 45-year-old Olivier Sarkozy ... didn't quite go full-Zellweger, but something's up.

Hey, at least we can finally tell them apart.