There are no Original Ideas in Editorial Cartoons!

I winced when I saw the above cartoon on Cagle's Cartoon Index because it was followed immediately by this one:

This is exactly what every editorial cartoonist lives in fear of: that someone, somewhere, has had the same idea as you. The thing is I would guarantee there has been no plagarism here: if anything drives a cartoonist it is to be distinct by virtue of originality. This is an example of convergence.
There is a finite gene pool of tropes, references and metaphors that is at the disposal of an artist, and an idea (and don't tell anyone this because it's a trade secret) is simply the recombination of elements from that gene pool in the artist's head. And sometimes this limited gene pool mixes a remarkably similar idea in the heads of two different artists. This is a really good example of where the refinement of the idea, the composition of the image, and the execution of the gag come into play: I would consider one of the above to be more successful than the other. Read back over them and I think you'll see one of them has a sharper delivery complemented by a good composition.
I would imagine there was mutual head-slapping in Minnesota and Detroit over this, but sometimes convergence is unavoidable. I submit to the jury this example, and also my hilarious take on the bird-flu panic (I'm really flogging a dead bird at this stage, that's enough H5N1 references for now!).