When a guy puts on a strap-on dildo and proceeds to fuck a girl in both holes at the same time. The strap-on dildo must go above the real penis.
True Double Duty is pulled off when the dildo is in the vagina and the penis is in the anus when the girl is on her back.
Philip "I took this really awesome girl home from the bar last night."
Jeffery "Ohh yea? Why was she awesome?"
Philip "She let me go double duty on her. She loved it!"
Jeffery "That is awesome."