I'm turning into my professor - the famous professor H. I once helped the man move... I didn't actually carry anything. My whole job was to sort the books by "period" and "theme" (or sub-theme of history, such as religion, warfare, mentality, nobility). It took me a whole day, long into the night. While I did that, I noticed that he owned one book (Montaillou - a book on an inquisitorial investigation in southern France during the late Middle Ages) FIVE times. Three hardcovers and a couple paperbacks.
I pointed at them, as they sat there, lined up on the shelf and asked him "why?"
He said that one had been a gift, one was owned by his wife, he bought the other one since he'd forgotten he owned the book, and the fourth was bought when he couldn't find it.
Yesterday, I bought a book because I couldn't find an absolutely crucial bit of information on German horsebreeding in WWII. Very soon, I'll enter the stage where I forgot I owned stuff (when I moved house, I already found two duplicate books - one pair on creative writing, the other pair on the history of Islam - so we're not far off).
I do wonder if ebooks are the solution to that. After all, it should be easier to organize them. Especially when the vendor keeps a copy on the shelf.
But now I'm eagerly awaiting the delivery of my horse book. And once it shows up at my door - I'll just pretend all this didn't happen and either give the second copy away as a gift (my professor gave me one of his five copies after a thoughtful moment and a "guess I really don't need FIVE."), donate them or sell them on.
Old age will be fun.