1996/1997 film written and directed by Herb Gardner, starring Walter Matthau, Ossie Davis, Martha Plimpton, and Craig T. Nelson, produced by Gramercy Pictures.
From MovieWeb:
It's the funny and exhilarating story of the unlikely bond that develops between Nat (Matthau) and Midge (Davis), two lively denizens of Central Park who come to rely on each other throughout their various escapades in and around New York.
Nat is a trickster who has a different false identity for every occasion. He still lives by that old vaudeville routine that says "I'm Not Rappaport."
The "I'm Not Rappaport" routine makes a great verbal game. It goes like this:
Nat: After everything I say, I want you to say "I'm not Rappaport."
Midge: I'm not Rappaport.
Nat: That's right. Now here we go.
beat
Nat: Hey, Rappaport! I haven't seen you in ages. How have you been?
Midge: I'm not Rappaport.
Nat: Rappaport, what happened to you? You used to be a short fat guy, and now you're a tall skinny guy.
Midge: I'm not Rappaport.
Nat: Rappaport, you used to be a young guy with a beard, and now you're an old guy with a mustache.
Midge: I'm not Rappaport.
Nat: Rappaport, how has this happened? You used to be a cowardly little white guy, and now you're a big imposing black guy.
Midge: I'm not Rappaport.
Nat: And you changed your name, too!
This routine can be tailored to the particular characteristics of the person you are performing it on. It is easy to work in some playful insults, but cooperation is critical to the punchline. Don't let them get away with saying anything other than "I'm not Rappaport." | 
