I've posted this before and I will do it again:
Both Duck and Duct are equally valid names for the magic silver tape.
In fact, it was ORIGINALLY called Duck tape.
Story: I think its WWI, some genius invents it for the naval infantry (no USMC at this point) and it gets the name 'duck' tape because water runs off of it like off of the back of a duck.
Of course, everyone who saw it wanted some of their own, and it quickly spread throughout the entire military. At some point when its use in ventilation ducts was widespread people started calling it duct tape.
Most companies print duct tape on their rolls because it sounds more builder-like.
They both are right.