One day, when a speaker was introducing logician and puzzlist Raymond Smullyan, he remarked that “Professor Smullyan is unique.”
Smullyan, who was feeling playful that day, said, “I’m sorry to interrupt you Sir but I happen to be the only one in the entire universe who is not unique.”
On another event, Smullyan was introduced by his former student Professor Melvin Fitting at a mathematics lecture with this: “I now introduce Professor Smullyan who will prove to you that either he doesn’t exist or you don’t exist, but you won’t know which.”
This was a reference to one of the proofs provided by Smullyan in What is the Name of this Book (1978) which said that either Tweedledee or Tweedledum exists, but there is no way to find out which.
Raymond M. Smullyan, The Gödelian Puzzle Book: Puzzles, Paradoxes and Proofs, 2013