Sometimes, there are cartoons that are deja vus. Great minds think alike and all that.
I remember talking to a cartoonist who, once he wrote down all of his ideas for magazine cartoons that week, he logged onto Cartoonbank.com to double-check to make sure that none of his cartoons were unconscious copies of previously published cartoons.
Regardless, there are times when magazine gag cartoons repeat themselves.
Obviously, if you think that the Great Cartoon Idea you have is not really most likely YOURS, then you should not draw it.
I try not to worry about it. Duplication happens. Even the places that can afford an entire fact-checking department, like The New Yorker, can't get it right.
For instance, you type in the words "cat heaven" into the Cartoonbank.com search engine, you will see identical wordless cartoons of a cat entering the heavenly gates via a small cat door. One was in the 7/22/91 issue, drawn by the UK cartoonist Kenneth Mahood, and the other in the 10/14/02 issue, drawn by Gahan Wilson.
Harry Bliss has a cartoon in the new Playboy that is exactly like a pretty famous editorial cartoon of Pulitzer prize winner Clay Bennett's (Clay drew Clinton wearing an "I'm with Stupid" t-shirt, with the pointy finger pointing down; Harry Bliss drew the same shirt on a jaunty lookin' dude).
And then there's Zach Kanin's cartoon in a January 2008 issue of The New Yorker that is an exact echo of a cartoon Don Orehek drew for Playboy a couple of years ago. (This one has a woman looking on as a plumber bends over -- and we can see he's wearing frilly underwear.)
I don't know for sure is these are intentional swipes. Only the person who drew the cartoon the second time could tell you.
Sometimes, though, you know for sure, deep down, that a cartoon has absolutely been pilfered. For instance, here's a 1952 Virgil Partch cartoon from True Magazine:
And here's one from the 1963 Cartoon Classics booklet:
Same angle, same phone, etc. as the later drawing above.
It's a rip off. You know it like you know a good melon.
Or rather, a rotten melon.