Dupes work a little different now. You must use scrolls, and the trick now changes the amount of your dropped item to the amount of scrolls you have (e.g. 30 scrolls = 30 of X item) If you have more items than scrolls, it will not work. You can drop all but one of the item your trying to dupe and pick the rest up when your done if you really need that much more.
Here's how you do it:
1. Select your scroll of choice TWICE
2. Drop your item of choice
3. Exit your inventory