I killed the duke by running up a hill scared like shit when I did it the first time i fighted him and when he got there I didn't know what to do...
So I jumped of the cliff and survived, I used my chance to get away and a few seconds later I heard the sound you hear after you killed an outlaw.
He jumped of the cliff
*edit* and btw I think you'll get the amount of moolah for 'captured him dead'...