This was super-fascinating to me! Obviously book designers often use stock photographs and common motifs, but here are two blogs which systematically compile and showcase trends and copycats/similarities in cover design, including some which even use the exact same stock photos.
I find it hilarious that you didn’t just find A blog about duplicate book covers, but TWO blogs about duplicate book covers.
And here’s another, albeit without the handy tag/category view…
Hooray for iStockphoto!
Ironically, one website about copying is a copy of the other…
Has anyone else noticed that the cover for Machine of Death looks almost exactly the same as the cover for “A Scientologist’s Guide to Alfred Hitchcock (illustrated edition)”?
Friend of mine used to work for a literary agent. They didn’t often represent fantasy authors, so she had a bit of a problem explaining to the agent that the “original cover art” on one author’s new book was anything but.
It consisted of a guy on a hill, with no shirt, waving a sword over his head…
I think you’ll also enjoy Caustic Cover Critic, especially posts like this:
http://causticcovercritic.blogspot.com/2011/01/caustic-kindle-critic.html
A book I helped write did the same thing, there was a long debate with the publisher over if they should use the image for the cover or not.
Making Light played a few rounds of Match The Covers a while back.