My ex is a master of disguise?!?

So at the bar last night, a man (whom I have never met before), started flirting with me. I got home and started to get into it. I noticed that he had the same birthmark on his stomach as my ex did. When I confronted him about it, he took off a mask and it turned out to be my ex! What should I do to prevent this from happening in the future?!