This duplication cheat may require a few attempts, depending on where you tap on the screen (see the directions below; you have to get it just right or it won't work). Note that the exploit seems to only work on the small and possibly the medium packs. The larger pack won't allow the item duplication.
1) Fill the loot chest in your safe house until you can see the very top right-hand slot
2) Place the item you want to duplicate in that slot
3) Check your backpack
4) Tap to the right of the top right slot in your backpack (as if there is another slot next to it)
5) Your item should glitch its way into your hand and you can place it in the backpack