Not sure about your type of duck but, we had the same problem last fall with our mallards.We had 2 male and 2 female who were raised together and lived together all year with no problems at all until october hit then bam! The males started fighting something firece and would not stop no matter what we did.Mating season oct - mar.Yuk! We finally seperated them 1 male and female in one pen far away from the other male and female in another pen.Which worked out fine.This year we are faced with another problem those ducks babies who are now into the mating season and starting to show signs.The females get along fine with each other.hope this helps!