This weekend I kicked off a bookshelf cleaning in my house. We're not done yet - our house has three bedrooms, which contain between them 12 5 shelf IKEA book cases, and there are 5 longer standalone bookshelves, as well as miscellaneous other spots where books are hiding.
But so far, we have learned that we have 5 copies of Treasure Island, 2 copies of Robinson Crusoe, 2 copies of The Two Towers (only one of the other LotR books), 2 Foundation's Edge, 2 The Secret of Platform 13, 2 Trinity (by Leon Uris), 2 20,000 Leagues Under the Sea, 4 copies of Call of the Wild (7 if you count it's presence in short story collections, including 2 Tales of the North), 2 copies of Jonathan Livingston Seagull of all things and probably other duplicates that we haven't found yet.