What if someone shows up in the same dress as me?
If somebody shows up in the same dress as you, you should kill them. No, seriously, what you should do is handle it cool. Most likely this is not your friend, because most likely if it's a girl most likely you went shopping together or she's already seen what you're wearing. So it's somebody you don't know. Play it chill. It doesn't matter. She's not going to be in your pictures, most likely. Nobody's going to know. So just have fun, don't pay any attention to it. It's completely irrelevant and 10 years from now you're just going to remember what you wore, not that somebody wore what you wore.