This may be an issue with mcMMO itself, however whenever a player breaks a protection block, they have the chance for double drops, and since they get the original block back, they can then drop it effectively an easy dupe.

Hi I got this problem as well!
do /ps on place a redstone ore protection field, use a pickaxe to break it and you get a dupe! this might infact dupe all blocks now -_- ..

Latest version of mcmmo tracks all player placed blocks and gives no xp/double drop for them. Update mcmmo