Doppelbanger

Noun: A person that looks so much like your current boyfriend or girlfriend that it could not be considered cheating on them if you were to be caught (or not).

Someone that resembles your current or past relationship that you may have the urge to call them by the wrong name.

A person that looks so much like a celebrity, you may just get with them because of it.

"That chick he was with looked so much like Anna Faris, it could have been her Doppelbanger"