Basically, when you're fighting the boss, you can Shout the sword off the pedestal. Then you pick it up off the floor before clicking the pedestal to complete the quest. You wind up with two copies.
Because I have a tendency to dual wield swords (and was doing that the first time I accidentally duped the sword), I wound up wielding them both for a while.
The only downside of it is that it doesn't temper right. And that the undead explosion power can be a problem if you're using Serana as a follower.