I have a problem with some NPC's getting out of control item duplication. It happens with someone I share inventory with and they have a negative number of an item in the CS. I have a person unable to move because they have 1536 bread loaves. I don't want to give up the option of sharing or edit each npc in the game to stop the effect. That would take forever and get ruined on updates.

Does anyone know of a mod that stops this behavior?

Find the item's editor ID, and use "AddItem [ID] -[amount they're carrying]".

Alternatively, remove all of the respawning items when you trade items. Repeatedly do that until no more are left.