Hello I’ve been running into this issue for a few weeks now since I’ve started farming old raids for mounts. I’ll do ICC/Dragon Soul/Firelands ect and kill a boss, but only getting like 2-3 pieces of loot to drop and will have to reloot the boss to pick up 1-2 more pieces that are duplicates of pieces that’s already dropped it’s never anything different. Basically this makes farming out mounts to feel nearly impossible because I think those loot slots are being taken up by other pieces of loot that are duplicates. Has this been happening to anyone else?
Actually I ran into this. It hadn’t occurred to me that it was a bug. This seems to happen now and then in heroic dungeons too (like Utgarde Pinnacle).
Mount drops are always separate. As are legendary items, where applicable (including the capacity for Illidan to drop both glaives simultaneously).
Duplicate drops isn’t really anything all that uncommon. It’s been happening since they were current content, and will probably continue well into the future. That you have to double-loot them is a little odd, though.
I wonder if it’s the same phenomenon as gathering professions often having to gather twice from the same node?
I have noticed the effect (rarely) looting normal mobs out in the open world, too. Usually, with duplicate items. 
Still, dupe loot from legacy raids is a common effect. When I was farming Dragon Soul 25H, Ultraxion dropped two copies of the same tier chest token I couldn’t use, two copies of a ring I didn’t need, and Experiment 12-B. So dupe loot was clearly not an issue.
OP, it’s natural to try to fix blame for unsatisfactory loot odds on a “fixable” bug. But mounts and other rares are actually rare. And that’s no bug.
To me the duplicate items isn’t an issue I know things can duplicate from boss drops but it’s having to loot the boss again to pick up the duplicate items. It’s like doing Thok last night I had 4 pieces drop at first 1 being the Tail Tip. Then looting it again for the last 2 drops 2 more Tail Tips. The reloots are always an item that’s dropped and that makes me feel like something is up.