I accidently discovered how to easily duplicate items. I think its a temporary duplication, but I haven’t tried creating legendaries, crafting on them, or changing the items to see if its abusable. If you just strictly duplicate it without changing it, the item is just temporary until you log out. The duplicate item appears to work just like the original.
It’s pretty simple to reproduce. Simply fill an entire stash tab full (minus a couple empty spaces). Put the item you want to duplicate near the bottom left nearby the sort button. Click sort, and very quickly (while the system is calculating how to sort the items), shift right click out the item you want to duplicate. The re-sorted inventory will contain a duplicate copy of the item that you just pulled out.
If you relog, the duplicate version (not the one you pulled out) will either (1) disappear or (2) change into another item that was in the stash tab with the original item in the stash tab disappearing.