1. Get two of them. 2. Junk them. 3. Sell One. 4. Hit Y and then A really quickly on the one that is still in your inventory. 5. If it worked you should have 3 in the shop. 6. Repeat 3-6 7. ??? 8. Profit
OH and so you don't start another topic because there's a couple already search for how to unglitch the tomes so you can use them. Because when you dupe them they become "stuck", but there's a way to fix it.
there is no trick just dont have more than one type of tome in your inventory. Like if you have physic tomes you cannot have mage tomes and just use one of them you have to store or sell the other ones then you can use them. also if you use a tome from your inventory you will need to back all the way out before you will get the points.