For those of you who want to abuse this, here's a guide:
0. (This one's kind of obvious) Put all items you want to dupe into the rightmost Fan's inventory. Click their name at the top of your screen to enter their inventory.
1. Go to the sewers(footnote 1) and enter them at any floor. If you are already in the sewers, exit them.
2. After entering or exiting the sewers, find the rightmost Fan at the very top of your screen, NOT the item trade menu, and click them to enter their inventory(this ensures you do not fail the dupe if you have a lot of Fans or messy inventories).
2.a. Now you may open the item trade menu from the rightmost Fan. Click into the other Fans' inventories from within the trade menu.
3. Empty these Fans' inventories into the rightmost Fan's inventory.
4. Repeat!
Footnote 1: The dungeon does not matter, but the sewers are the safest, additionally, you can save, quit, and reload instead of using a dungeon, but this can take longer. This is useful if you have temporary buffs that remove themselves on exiting a dungeon.