Dreamfight scenarios are always so complicated, as boxers are different every time they step into the ring. But, if you were to take each fighter on their proverbial best day ever, I think Duran knocks Mayweather out in 7.
Mayweather is undefeated because no man can catch him (yet). I think that Duran was the man fast enough and tough enough (to take the counter punches) to knock out Pretty Boy Floyd. I think that it would end with a KO in round 4. Duran was a pretty good hitter too. One of the only people to ever beat Leonard. Leonard can beat Mayweather no doubt.