Moving the thread to
World Editor Help Zone, because the thread is not about your code/triggers.
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There's an action to manipulate and to get an item's charges, so removing both items isn't required.
When a unit picks up a new item which has charges, you have to check each slot in the unit's inventory (Triggering Unit) for an item which has the same ItemType. If yes, you can add charges to this item, and remove the picked up one.