Do you have multiple copies of any of your books?
If so, why? Absent-mindedness? You love them that much? First Editions for the shelf, but paperbacks to read?
If not, why not? Not enough space? Not enough money? Too sensible to do something so foolish?
I have multiple copies of a few Dickens books, because in addition to modern paperback editions, I also have some early editions and some specially illustrated editions. I've also bought different modern editions of the same book so I could see different introductions and different end notes. In short, if you ever want to come to a party where the main activity consists of reading Bleak House, I'll be your hostess with the mostest!
I also have multiple copies of a few historical novels, mainly because I have a weakness for tacky cover art and saw a cover I couldn't resist.