I have two copies of Alberts' The Molecular Biology of the Cell.
If so, why? Absent-mindedness? You love them that much? First Editions for the shelf, but paperbacks to read?
I suppose on first glance, it is sort of crazy to have two copies of Alberts. The thing is like a concrete door stopper. Not to mention, expensive. But if you're in my field, you have to invest in these kind of things. I got my first copy as an undergrad. However, when I went to grad school, they came out with the next edition which had some new stuff in it. There's always new stuff coming out in science and, well, you just have to keep up. Unlike 18th century British literature.
If not, why not? Not enough space? Not enough money? Too sensible to do something so foolish?
Why I do not have multiple copies of my other books: For most books, I don't need multiple copies. I'm not the sort of bibliophile who enjoys collecting books for the sake of collecting books. I buy books because I read them. I want the story or the information, not the material stuff that it's housed in.