This phrase was brought forth via R. Kelly's "I'm a Flirt" single in which one of the lyrics states, "she'll be calling you Kelly when your name is Tommy." The term applies to certain individual states during which an individual exists as both Kelly and Tommy simultaneously, particularly during intercourse. At the same time, the phrase also applies to general states of confusion regarding one's identity.
Tommy: Hi, I reserved a table for two for 7:30
Host: let me see... Kelly?
Tommy: actually, my name is Tommy but I can see how one would confuse me for Kelly as I am going through an identity crisis and as such exist in both Tommy and Kelly states simultaneously.
Host: Naturally, I apologize for not recognizing the Tommy-Kelly entanglement sooner.