Alright so, i chose to write this in general and not in exploits since i am really not sure if i was just lucky since this requires a GM. I wanted to share it even if it seems a bit confusing and perhaps really hard to reproduce. I wasn't even trying to dupe the mounts when this happend.
Alright so it all started with me buying an account from someone, i played on this account for a while and everything was great but due to me being paranoid of losing the account i proceeded to by a Batte.net name change from BSMunban(which btw the way was great and can definetly reccomend his service!). The name change was succesful and my first name and last name were changed in no time. Since i now had my real name on the account i thought to myself, why dont i try to recover my old account which i knew had my name and i also remembered the name and server of the main character on that account(I hadn't used this since WotLK - note that it was merged to another battle.net account)
So i wrote a ticket from my newly namechanged Battle.net that i wanted to recover my old account and if it was possible if they could merge it to my account new battle.net. I told them it had been hacked and i really wanted to get it back. The next day i found my old wow account was merged to my new Battle.net so now it had all the mounts from the account i bought(Black proto, Swift Alliance steed etc.) Since all wow accounts on the same battle.net share mounts.
It was all good for the next month until one day when the OO of the account i had bought chose to recover his Battle.net(which he did even though i had namechanged it) this resulted in all the accounts being locked while blizzard were looking into it, i wrote a ticket asking them why my accounts were locked and they told me about the situation so since i knew this ment the OO was trying to recover it i gave up and told them yes i had bought the account, but i still had my own Legit account on the battle.net and the told me to create another battle.net under my name and then they would merge my old wow account to that.
To my surprise all of the really rare mounts from my bought account were still on my account after it was moved! (they were also on the account i bought, i checked armory)
I only got the mounts not all the achievements except the feat of strengths that the rare mounts provide.
I know this seems pretty confusing... If theres any questions ill gladly answer them, as finding a way to easily reproduce this could be big. I haven't tried to reproduce it myself, but maybe someone could use this to figure out how to dupe account shared mounts since atleast this proved that you can still dupe them even after the "fix"