A) A pejorative for a Roman. This term has its origins in the phrase "Dick Duck Fart"--a Roman motto (dic duc ferte) meaning say, lead, and bear. Rome's subjugated people referred to their conquerors as "Dick Duck Fuckers."
B) Possibly apocryphal texts suggest that the term had its origins in an incident involving a legionnaire and a Gaul. Like all Romans, the legionnaires would march through enemy territory chanting "Dick Duck Fart." As one especially savage soldier chopped a baby's head off, the mother screamed: "You're all dick duck fuckers!" making a mockery of the Roman battle cry. She was subsequently drawn and quartered.
A sexual position, found commonly in Oregon, where the male party drinks the female's vaginal liquid discard, then pisses it back inside the female party. The male then ejaculates inside the female's mouth and she shits it back into his mouth. Thus completing the two headed duck fucker.
The human sexual activity that is analogous to the coital actions of ducks in which the male mounts the female in a very domineering manner. He grasps her neck with his bill and submerges it beneath the water. This would be analogous to a man forcefully shoving a women's head or neck into the supporting surface while fucking her from behind.