Hey I just posted this below, but I wanted to officially confirm this new way of duping on private servers. Granted you can only dupe 19 items at a time, but still, better than nothing
1. Make/Buy Deadly Poison VI
2. Make/Buy a stack of 20 Deadly Poison IV
2. Right click Poison then drag to item you want to dupe. (to wrap)
3. Drag your stack of 20 on top of the wrapped item.
4. Shift-Right click the new stack, and select 1, then put it in empty slot.
5. Notice you now have 19 duped items to sell to the vendor
All credit go to me, I really think I'm the first one to come upon this :P