If I've said it before, I've said it 1,238,647 times--repetition is a vital element in humor.
I will even go one further and say it is in fact essential. Repetition--mixed with a tasteful amount of variation--is key to creating narrative. Repetition brings both familiarity and jumping-on points for listeners/readers/whatever. And it adds to the general comedic effect. Regardless of what some people may claim, if you repeat something often enough, anything can become funny...or funnier.
General repetition provides possibilities for improvisation, for exploration. One classic examples is Bugs Bunny's catch phrase "what's up, doc?" On its own, it was maybe slightly funny for its incongruity (coming from a rabbit and all)...at least during its first few utterances. With repetition, though, it becomes a trademark. Its value is not, after countless repetition, due to any essential humor contained therein. But "what's up, doc?" sets up the viewer for Bugs Bunny's patented style of hijinks. As such, it gains a certain amount of funniness via association with Bugs Bunny, with the context of forthcoming laughs.
Yet it also provides an opportunity for riffing. In the classic 1946 "Hair-Raising Hare," Bugs is being chased by a giant, scary monster. He's scared, he's breathless, and generally out of control. But then:
This chance to use the oft-repeated phrase "what's up, doc?" allows Bugs to calm down, to munch on a carrot, and generally regain his confidence. For Bugs, it's an incantation as much as anything else. For us, though, it is humor from repetition. The cartoon introduces the familiar (and not for the first time) yet modifies it enough to subvert our expectations...which produces humor. This entire bit, in other words, is set up by the regular repetition of "what's up, doc?"
Repetition sets up expectations in the audience which can then be subverted by the text . Whenever one uses repetition, it establishes in the audience an image, a trademark, an expectation. These are all perfect fodder, perfect prompts for comedy...or, for that matter, drama, insight, whatever.
I remember once (decades ago) seeing Jay Leno on David Letterman's show. Leno (who was a much better stand up than tv host) was on a rant which led into the old (ancient, prehistoric) joke about how nowadays in college dorms, you could have sex, you could have drugs, but you still can't have a hot plate. It was awesome...not because the joke is funny in and of itself (let's face it: the joke stunk when new) but because the joke had been repeated so often (run through the wood chipper, actually) that it became funny again as an anachronism.
Then there is Bugs Bunny's repeated use of "what's up, doc?" While this was incongruous enough to be funny the first time, as he continued to repeat it, it transformed into a crutch to set up other humor. Sometimes, it became a great line to be recontextualized and played with, such as in 1946's "Hair Raising Hare."
One of the more classic Monty Python sketches was "The Spanish Inquisition"...which was repetition and variation throughout the night, repeated and repeated, expanded, played with, and eventually ending the program in an aborted version.
Yeah, a lot of the humor here is from the general incongruity, but each repetition keeps you on the edge of your seats, thinking "where will they go with it this time?"
Another example of repetition setting up improvisation is in the 1946 Bugs Bunny cartoon "Hair-Raising Hare." Here, every single past repetition of Bugs's famous catch phrase "what's up, doc?" becomes the backdrop allowing this particular story to use the line as a jumping-off point for new comedy.
One example of repetition which doesn't get nearly enough recognition is Saturday Night Live's amazing coverage of the assassination of Buckwheat (and then of the assassination of Buckwheat's assassin).
The repetition allows SNL to perfectly rail against news coverage's own reliance on repetition in the absence of new facts and ideas all the way back in 1983...long before The Daily Show existed.
And then...hey, have I told you about how Bugs Bunny uses repetition?
The most important--and undoubtedly the most essential--feature of repetition in humor (and in writing in general) is narrative. Quite often, I am lucky enough to get pieces come in to my mind with a pretty good structure. More often, however, I look back on my rough drafts and see a bunch of random observations nailed together with beer and ink. You've seen this phenomenon a lot in many venues, but it shows up quite frequently in stand up comedy. Lots of comedians have jokes. Sometimes, though, they seem to follow a Zapp Brannigan-esque approach: "say as many of them as possible, as quickly as possible." The problem, however, is that jokes are not the same thing as an act...any more than a series of observations, gags, and one-off lines (such as Bugs Bunny's "what's up, doc?) is a coherent story.
Repetition, though, can add cohesiveness. Hit on a neat idea somewhere in your piece? During rewrites, it can become an element in the title. It gets mentioned a couple times in passing. And then, when you're specifically looking for a narrative conclusion, you work your way back to the original point. This became really clear to me after reading WWDN, but you can probably think of a million other examples at this point.
Oh, and have I mentioned Bugs Bunny yet? Just checking, because sometimes, repeating something over and over and over and over can work as a complete and utter substitute for narration. Don't believe me? Then would you believe you're already at the end of this piece?