This method requires at least 2 people, but would be easier to preform with 3.
Character A is the inventory to dupe
Character B is a player wearing nothing at all
Character C is a player who has both hand cuffs and a handcuff key

1. Have A log out of the game
2. Once A has left C puts the hand cuffs onto A's log out entity
3. Character B then accesses A's inventory and moves all the gear to his own
4. Once B has taken all the gear, or about 15 seconds have passed; C uncuffs A

I theorized the other day that character data is saved when the player exits the game, and not when their combat-log entity leaves the world.

Originally I tried cuffing the player and grabbing some gear, but did not uncuff the player before his entity was removed. This resulted in him spawning as a new character the next time he joined(The combat-log entity was removed from the world and never created a corpse; there are other reports about this)

On the second test we had a third member handle cuffing and uncuffing; We were successfully able to take the entire inventory of the member this way, and have him spawn with all his gear in tack; duping it.

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This method of duping will continue to work for a long time, unless character saving happens after the combat-log entity is removed; or you remove the ability to cuff a logged out player.