Unfortunately that’s become the “Play it again, Sam” of my generation.

The joke comes from the movie Johnny Dangerously and it’s in the form:

“X did something to me once… ONCE.”

It only works if there are two “once”. It’s supposed to tacitly imply that the person did it to the speaker only once because then the speaker did something horrible to the person and so he/she was forever unable to do that thing to the speaker again.

If you only say “once” once, it doesn’t work!!! It’s not funny. It’s just stupid. It only works because it starts sounding like the speaker is merely relating an anecdote (X happened to me once), and it’s only the repetition of the term “once” for emphasis that turns it from merely a random comment into an admission of murder, which is always hilarious.