If you showed which books I have already bought it would stop me making multiple purchases of the same book (thus getting mad at me and your site) over time.
I have a reasonable sized library and cannot remember all the titles I have bought over time.
Amazon does this.
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So sorry for your frustration. On oreilly.com you do have a list of ebooks/print books and videos you can reference before you make a purchase. If you log into your O'Reilly account please click on "Your Products" which will have these lists.
If you are ever to purchase an ebook twice we would be happy to get another ebook of your choosing as a replacement for the ebook you have, or if you have no ebooks in mind we could give you a refund. Just contact us at firstname.lastname@example.org or call us at 1-800-889-8969 or 1-707-827-7019 from outside the USA 7:30 am to 5:00 PST Again, sorry for the inconvenience this may have caused.
Customer Service Representative