I don't think it is imprinting. It's normal drake behavior in adolescence in the spring and likely every year for the next few years. My flock is all drakes and they have started their drake nonsense already -- I had hoped to have some peace through January!!!
Mine have started their pecking order behavior and there has been a change in the order. I caught Daffy, who is top of the pecking order all the time except when he is off color/molting, standing on Butter who was bottom of the pecking order and ignored by Daffy previously. Then I realized that Butter is now senior to O Pato. Meanwhile Daffy is biting my ankles and shins [hard] trying to tell me he is my boss duck too, He is not, and daffy gets squirted with the hose if its in my hand and picked up and dumped in the wading pool if the hose is not to hand. I have an ongoing issue with O Pato attacking my feet from behind and he to is dropped int he wading pool -- he ates water.
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Naiva80512 best to manage the bullying be separating the two drakes at night. I use dog crates in my duck house to separate my ducks at nights. But any other way of partitioning their sleeping space would likely work. I find the main bullying goes on at night in the confines of the coop. During the day mine free range and there is enough space and shrubbery for the warring parties to lose each other. However, if you have a small run for your ducks, you might have to partition the run too, to stop the bullying. Mine have always had the pecking order bullying for 3 or 4 months in the spring with a shorter recurrence later in the summer.