There is an excellent second-hand bookshop near me. I'm always impressed by how they stack the books in the non-fiction section using the Dewey Decimal System!
Anyway, whenever my bookshelves start to get a little crowded I donate some of my books to this particular shop.
Needless to say on numerous occasions I've gone in there and unwittingly bought back books I've previously donated
It seems to be getting worse the older I get