I used to know how it was done, but it has been fixed it now.
It was very similar to the DSoma bug where you could be able to buy pots whilst out hunting.
What you needed to do was:
Make a new character and stand them by your warehouse.
Log onto your noobie character and press Ctrl+Q (it won't work if you use Alt+F4). When the logout timer reaches 1, VERY QUICKLY press F12. It will still log you out, but it will also hold the F12 screen up.
Now change characters to your main character who should also be stood near the warehouse. If you have done it correctly so far, you should see the chat window open. Don't close it. Click on the warehouse man and it will change the chat window into your warehouse screen. Take the item you want duped out of your warehouse and logout again.
Now both your characters will have the item for some reason. Keeping the F12 window open somehow (I think) keeps your some of your noobie's information around whilst on your main character, so instead of just giving you the item, it gives it to your noobie too.
Remember, this doesn't work anymore :p So don't even bother trying :p
ya i used 2 loving using that bug at train i never went town lol
i used gs,repair,wh,pot seller i only com town for guild war but now its just !@#$%^&*
there wa also the one wen server was gonna go down .. just before it went u type /save then give the item u want to dupe to someone then let the server kick u off... come back on u both had items