It's the celeb version of the ultimate prom nightmare: some bee-yatch showing up in your dress. Sure, Alicia Keys' and Ashanti's Grammy getups weren't identical, but the peaches-and-cream Roberto Cavalli creations prompted double takes on the carpet. And in an industry where stylists are paid big bucks to make sure their clients don't share the spotlight, the double-the-pleasure moment is a big oops. ''I don't think the girls knew they would be wearing the same thing,'' says a Keys spokesperson. A Cavalli rep says the designer had been working with both stars for the event.