I actually had 3 of them. Only thing I can think of is I had 4 toons at 80+ and 80 being the lowest lvl you can be and still get the account wide achievement credit (due to Naxx being 80). So it was bugged and not only gave the achievement to all the toons but also the pet.
"Even when you are winning soundly, you must always give your opponent a way to retreat with honor. If you don't, then he has no reason to surrender. He will fight until the bitter end, and you will pay a larger price for victory."