From what I understand it should protect you from getting a matching card unless you already have all of the items from that roll. I.E. If I had all ranged assault rifles in the epic variant it would duplicate one.
That is correct. The Llamas will roll based on your account so there is a chance that you can get 2 of the same items from the Llamas if you don't have them.
However, in your case.. are you saying you DON'T have all of the Epic ARs and Shotguns and you are getting a dupe? There does seem to be an issue with Dupe Protection and we're investigating that :)