Hi Michelle,
My drakes fight every spring but then they're in the same pen as the females. Not all the drakes fight they seem to pick on one or two. I have a pen of 4 males and they never fight, they are seperated by quite a distance from the females. Are your males and females out with each other during the day? I would expect that would be the reason they're fighting. I don't really think they'll kill each other but if they get too rough, seperate them until breeding season is well under way.
I hope peace will rein amoungst your ducks soon.
Ducks are my favorites!
Phyllis