God it really pissed me off! Twice now I've bought books that look good only to start reading, check the front and realise they were first published in 2006 or whatever. And I've already read them. Can't they have a little symbol on the cover or something?
Yes this is annoying. It usually seems to happen when a previously little known author has some success. Their (often new) publisher then dredges up the back catalogue and reissues them. I've come across some real turkeys this way as well as being misled into buying a book I've read before. I'm very bad at remembering titles when it's a favourite author who has written a lot.
Also annoying when they change halfway through a series so your books don't match. The wheel of time series had some great pictures on the first books then they changed them to a boring cover for the last books. WHY?!