Oh, it also does it if you switch tabs between characters that have the same slot of equipment filled (i.e both are wearing hats, the one that you switch tabs to has their hat duplicated as "unequipable" and placed in the inventory.) Any duplicates cannot be used by multiple people though, because when two people are wearing the same equipment, the first one to wear it gets theirs unequipped.
The only way to get these duplicates to go away is to equip a duplicate, close the bag, reopen the bag, then switch to one of the two who had the duplicated item on. If they're still wearing it, close the bag and reopen it to view the other person, who will now have nothing equipped in that slot.
I hope any of that made sense.