I did it almost twice now: I bought a book that I already have! So I have been searching for a program where I could scan the ISBN bar codes of all my books and let those go into a database. Well, I found the perfect program:
Book Collector from Collectorz.com
I bought the small laser scanner with it and started scanning the first shelf.
This is so much fun. Although the program doesn't find all my books (some of my quilt books are really old and not in print anymore and some front covers are missing), it is still worthwhile to get it all organized.
There's still a lot to be scanned, but once that is done, I can take the database with me and prevent that I buy the same book twice :)
(not affiliated with collectorz.com, just a happy customer)