To do this glitch have to have the Carry-On Mod (Let's you pick up animals + chests, barrels etc) + The Environmental mod. To do this dupe glitch you have to put a chest on a Slabfish then put the item you want to dupe on the FIRST Slot in the Slabfish Chest then pick up and drop the Slabfish multiple times.

I don't really know if you can fix this since you need another mod, just wanna put this here so yeah... ok...

Turns out that didn't fix it and actually made it worse (it dupes the entire inventory now, though technically intended), will have to investigate some more.

Seems like it's caused by CarryOn saving the entity into an item and then clearing the entity, causing the Slabfish to drop it's items while retaining a copied form in the entity item.