T. S. Eliot once said, "Immature poets imitate; mature poets steal." While it's hard to know exactly what Thomas Stearns had in mind when he said that -- if he were being brutally honest or mischievously playful (or somewhere in-between) -- but it's an undeniable fact that poetic forms, literary devices and, yes, stand-up comedy routines do get stolen.
In the realm of comedy, David Brenner publicly accused Robin Williams of repeatedly stealing his material, Denis Leary was accused of having basically ripped off Bill Hicks' entire act, and comedian George Lopez has accused Carlos Mencia of plagiarizing his material. Recently, even the talented and wildly popular Amy Schumer was accused of having stolen someone else's stuff.
Granted, technology has made the whole plagiarism enterprise infinitely more difficult than it was a few decades ago. With the ability to "research" virtually any gag on the Internet, comics are taking a big chance pretending to be original when they aren't--not when people can go on YouTube and see another person doing the identical material....six months earlier.
But it can also be argued that outright "theft" isn't as common as people think. Two jokes can be spawned by the same "mother" without either one necessarily having been copied from the other.
After all, didn't Newton and Leibniz both "invent" calculus independently of each other, with neither man knowing what the other guy was working on? If two mathematicians can independently invent something as complicated as calculus, certainly two comedians can independently plumb the same line. In fact, since both have "comedic minds," the odds are good that they'll eventually land in the same place.
I have personal experience. I invented two mildly amusing puns which were never used in any of my plays, were never published in any periodical, and were never told to anyone. Indeed, these two gags never got beyond being consigned to my "Idea folder," where they died of old age.
As I said, both were puns. The first was this: "When Jefferson Davis was in high school, he was voted Most Likely to Secede." I remember precisely where I was and what I was doing when I came up with that one. The second was this: "They're going to make sitcom about a radical young black kid living with a white family, called Malcom X in the Middle."
Here's the weird part. Long after I had written these down, I heard both gags being used by professional comedians. Obviously, they hadn't stolen them from me because there was nothing to "steal." All they did--and the more you think about this, the more sense it makes--was plow the same word-play ground that I was plowing, and come up with the same modest material.
I once heard a comedian appear to "steal" from Steven Wright. One of Wright's jokes was this one: "Yeah, sure, it's a 'small world,' but I wouldn't want to paint it." The comedian I heard did a riff on the old saw, "It's a small world," and said something similar to Wright, but not quite as funny: "But try paying for airfare." In any event, he and Wright were very likely exhausting the same resource. It happens.