I started the Cover Doubles feature because I was browsing through some books and had to do a double take because I just knew the cover I was looking at was the exact same cover as a previous book I'd read before. With digital self-publishing, stock photos and pre-made book covers are often used multiple times. But, I saw similarities happening with traditionally published books also. And that's what started me checking out cover trends.
Here's my fourth edition of Cover Doubles: