Look OP, this is how you do it. Next time you see "Amy" tell her that she looks just like one of your girlfriend's friends. Like, it's uncanny that you look so similar to each other. Drop it for 5 min then bring up again how much you they look a like. Then say, would you mind if i took a picture of you to show my GF?
Bam, you gotta pic. Not only that, but tell her that the next time you're you'll bring a pic of her "twin". But the next time you're in, you can sadly tell her that you and your GF broke up before you could get a pic of the "twin". Then ask her out.
"There is no greater tyranny than that which is perpetrated under the shield of the law and in the name of justice. -Charles de Secondat"