Interesting... I'm going to look into goodreads.
I have noticed that most of the books I read again (on accident) are ones that didn't strike me as being fantastic the first time around. Some books sort of blend into one another. I don't mind it so much when it's a book I did enjoy (although not super memorable), but it really irks me when it's a book I barely finished the first time around because I didn't like it that much. Luckily, I rarely buy books, so it's just a matter of returning it to the library again
or, more recently, clicking "return book" on my computer!