I don't buy it, wrt recommending something already purchased. If the one thing you know about someone is a specific book (or other purchase people tend to only make one of) they bought, it's a decent assumption that they are more likely to buy literally anything else from you.
I think some people buy multiple copies or order over and over again, so the actual probability is high relative to other items (because as others have said, the total number of items is huge).
I think maybe the real mistake is that someone who is buying a bunch of copies doesn't really need the item to be recommended to them. The recommendations are more helpful for things that you can't find in the history.
