This only works when playing online, I couldn't replicate it with local players. The player that is joining the game is the one that can duplicate.

• Open your present inventory and scroll to the present you want to duplicate.
  
• Quickly press the drop present button, then the open present inventory button, then the drop present button again. If you are fast enough, you will drop 2 presents from 1 that was in your inventory.
  
• Keep alternating between the open and drop buttons, if you are fast enough you may duplicate 3, maybe 4 or more before the present actually drops.
  
• The presents can be picked up and this can be repeated as much as you want. The only present that duplicates is the last one in a stack, so keep your stack low while dupping.

Below a screenshot with proof. (warning, may contain secret spoilers!)

Click to see image...