I did it on 1 private server so far, and it worked fairly.
But after a while they made it harder to dupe(somehow) and they made several items not stackable.
Anyways
This is how i did it:
1. Open 2 clients (don't have to be different comp or IP)
2. Open a shop on 1, press Alt F4 check your other client and time how long it takes before the shop actually closes and the toon dc's.
3. Re-open client, Open shop, put item in there.
4. Close client, and you have to buy the item just before the shop actually closes and the toon dc's
If you do it good, the toon that dc'd will still have his item, and the toon that bought it.
Will have the Duped item.
Now remember: Don't relogg your toon with the duped item. It will vanish after relogg.
So if you dupe: Goldbars,Op's,... use them immedialty before logging.
If you dupe gear, you'll have to do it 3 times in a row and then go to the BS and remake 1 unique gear.
Then it wont vanish anymore and it's not tracable by the GM's.
This method doesnt always work, you need some patience for it.
But once you get it, it's easy