Personally I think an identity is a conflict of two personas, one being the current persona and other being the persona one wants to become. If the two persona be the same, then no conflict arises.
Is it the way one looks, or speaks, is it one's demeanor or one's actions?
Is it all of the characteristics taken together or only few which are more prominent?
Is it the uniqueness of those characteristics or the similarity with some other known person?
I don't know, but maybe that's how it is supposed to be.
"It is not who you are underneath, but what you do that defines you", Rachel says to Bruce in Batman Begins. Of course later she realizes that there's more to identity than just actions!
I met a friend after about 8 and a half years and I realized that although she had changed in some aspects, some things about her didn't change, those things which I think defines her.
In Agantuk (The Stranger), a film by Satyajit Ray, the husband is skeptical about the uncle of his wife whom they have never met. Later the uncle himself confronts the husband, who seems to be convinced by looking at the passport about the identity of the uncle. Uncle argues that a passport could be fake as well, so is there a foolproof way of testing/judging a person's identity?
In the Christopher Nolan's directed Batman movies, I found Bruce contradicting himself.
In Batman Begins, Bruce talks about using a symbol as an identity for justice, to correct the situation in Gotham because a person could be bought or destroyed but a symbol couldn't be.
In The Dark Knight, Bruce feels that a person like DA Harvey Dent, a real person would give people hope and make them feel secure, rather than a masked vigilante and his symbol.The only reason that resolves this conflict could be that Joker, who was Batman's rival in The Dark Knight was again, not a person but a symbol, a symbol of joker, which stood for chaos and randomness.
P.S. - Geminis are said to have dual personas, as in they tend to contradict themselves often.