We show a warning on the book landing page if you already own the book. However, there are three reasons we allow duplicate purchases:
To serve as a tip jar, since we don't have one yet. Since our authors set their own price and get most of the royalties, we have numerous readers ask if they can pay more once they've started reading a book and want to pay more.
To buy a gift purchase.
To allow a reader to buy a different package of the book later -- some books offer multiple packages (say "the book" vs. "the book + code samples" vs. "the book + code samples + videos"). We currently have no way to upgrade purchases, so this is handy.
Readers have 45 days to get a refund, for any reason. So, if you inadvertently buy 2 copies of a book, please request a refund of one of them from your library!