Im not an expert on Game Design and Development so here's my take.
From what i've seen on how duping works is that you need a Sea Chest and to logout with it while turning Internet off on your device, possibly one fix is to make it so logging out with a sea chest makes it drop on countdown initialization or any storage object other than clothing for that matter.
Another idea is to have unique item ID's such as when a item is spawned it has the "ID" Attribute of "SERVER_NAME_ITEM-TYPE_ID-HERE" so no matter what server you are on (official) it won't coincide with another item's ID, where if 2 of the same items are found it just deletes one of both.
This was more of a suggestion on how to combat Duping so i don't expect it to be taken serious.