Wikipedia explains alter ego thus:
An alter ego (Latin, "the other I") is a second self, which is believed to be distinct from a person's normal or original personality. A person who has an alter ego is said to lead a double life.
So, alter ego sounds more like an identity disorder. But I remember reading somewhere that a person who thinks like you and has the same views and perceptions as you do, can be called your alter ego.
I would like to know if this term can be used to describe someone whose thoughts and behaviour usually match yours?