For some it is just a means of engage you in a conversation, especially when it is the opposite sex.. It is a cool way to actually getting a girl talking if she is the mood and into you.
On the other hand, it is very possible your look-a-like or doppelganger exist and the person is sincerely telling the truth. It has happened to me, that is people telling me, I am a prototype of someone else but I always tell them that the other guy is the prototype of me.
Also, I have seen a girlfriend who wasn't the person I thought she was.... I gave have hug and she pushed me away telling me she knows me not. It was an embarrassing moment. I apologized and moved on. Since then, I always wait for eye to eye confirmation before taking any action.