Crafting an item in an imbuement table while your friend repeatedly changes or removes the middle item for crafting can create a 2nd item.

Wow, that's quite the fun bug. What is an example recipe you've noticed this happening on? Are both items the "result item" or do you end up with one pre-craft middle item and one crafted?

I might simply make it so other players either can't look at a table that's in use or can't interact with the imbuing table inventory if another player opened it first.

You dupe the ingredient or the random item, the crafted item comes out normally. The recipe we noticed this on was with crystaline hearts