I've been planning on trying to get rid of some books to clear out space. My wife and I have multiple copies of several works, and I figured I would just sell those books we have copies of. Anything else, my wife, our child (or future children), or I may wish to read or re-read, so we should keep those.
But I'm finding that even when I have two or more editions of a particular work, there are reasons for me to keep both. If I have multiple translations of a work, I want to keep each different translation ( I now own three different translations of Crime and Punishment, and frankly, I don't want to choose which one to rid myself of). Sometimes a copy I might otherwise rid myself of has an introduction or something to make me keep it (Vincent Price wrote an introduction to one of my copies of Poe's short stories). Often I have one book I've marked up while reading, and another copy that is not marked up (I tend to mark up books, my wife doesn't). There's good reason to keep both editions--I may wish to find underlined passages at some point, but somebody in the household may also wish to read the book without any annotations (indeed, when I re-read Paradise Lost, I used a fresh edition so I wouldn't come across my notes from several years ago. I also bought a fresh edition of The Magus for future re-reading). And too often, there is some silly reason why I want to have two editions of a book.
A chance to really clear out some space has left me with a very small pile of books to sell, and all sorts of doubles that I just can't bear to part with.