It’s likely related to this: Have to herb twice
Though the title says Herb, there are several posts that mention looting in general as well. Old raids aren’t immune if a boss happens to drop 2 of the same item, not just Icecrown, but all of them (where identical items have a chance to drop).
It’s not so much a duplicating issue as a looting issue. It’s been going on since 8.2.5 dropped. I first noticed it with mining, but eventually saw it later that day when I gathered up about 7-8 crabs & AOEd them down. 1st time looting got 1 claw, 2nd time the remainder. It appears to only occur on the 1st loot of the mob for the day, because my next pull I only had to loot once to get all the claws.