Why do you think rollback dupes would result in only 2 copies of the same item? (or do you think that as you're implying it)
One person (or a team) can use rollbacks to dupe the same item over and over again. Most probably these items were duped months ago but they wont disappear because Blizzard can't just delete them or else they'd burn a lot of innocent players. Also rollback dupes explain the sudden crash of gem prices back in October where as traditional, continuous and "player controlled" duping does not.
I've yet to see any evidence that the duping we see would be anything else but rollback abuse. Thought I'm more than willing to hear any counter arguments.