There is a problem with duping items. if you have 50 arrows and you dupe a single ring (for example) you get 50 rings back, that's good. But if you try to dupe the same thing again and you have 50 rings and 50 arrows, it drops all fifty rings and you just pick up those same 50.
The solution is after you do the first dupe and get your 50 items, if you want to dupe it again and get more, drop all but one of that item, then dupe it and pick of the new duplicates plus the ones you dropped before hand.