Duplication Detection Question

How do mods know if an item is duplicated?

As of now, I don't know of a method other then paying attention to console and seeing if they all of a sudden have 2 of the same item, but not crafting it per there level. We have a console watch system. You can tell the console to watch a playing and it shows what they pickup. or have new the level and amount. So we can look through and see if they are not crafting or doing something that would normally get that amount or that same QL then we cought them Thats about it. There is no item id that you can check showing its unique or not.

That is all that I know so far? Anyone else got a way?