For my game, there's two types of items - stackable, and nonstackable.

Nonstackable items get assigned a unique ID that stays with it forever. A character ID is assosicated with the item, as is a state (CHANGED, UNCHANGED, NEW, REMOVED). The character ID and state is used for item saving purposes.

Stackable items have one unique ID, as in the entire stack has one unique ID. For example: 5 Potions (stacked ontop of each other) has one unique ID.

When dropping a nonstackable item, the state gets set to REMOVED, and the unique ID and state don't change. If picked up by another player, the state gets set to NEW, and the character ID gets changed to the new character's ID.

When dropping all items in a stack of stackable items (for example, 5 potions out of 5) - it behaves just like a nonstackable item.

When dropping some of a stack of stackable items (for example, 3 potions out of 5)... I really have no clue what to do. The 3 dropped potions have the state of REMOVED, but the same unique ID and character ID. If another player picks it up, it has no choice but to obtain a new unique ID, and its state gets changed to NEW and its character ID to the new one. If the dropping player picks it back up, they'd just be readded to the stack.

There's two issues with that though. 1. If the player who dropped the 3 potions picks it back up, there's no way to tell if they legitimately dropped the items, or if they're duped items. 2. If another player picks up the 3 potions (assuming they're duped), there's no way to know if they're duped or not.

My question is: How can I create a system that detects duplicated items for both nonstackable and stackable items?