Step 1: Open your inventory.
Step 2: Click spawner and move it to your third inventory slot.
Step 3: Move the item(s) that you want to dupe to you first slot.
Step 4: Move the spawner from your third slot to your first slot, and the item(s) from your first to the third.
Step 5: Log out and then back in.

Your item(s) should now be duplicated. ^-

I found this around 2 weeks ago and I had a plugin dev send it in, but it still exists in 1.6.1

For video on how to do it here's a link, the link is unlisted and may only be viewed by people that click the link.
http://www.youtube.com/watch?feature=player_embedded&v=ALE1x_ddJ98