The 40-year-old Duncan, a 17-year veteran, was assigned to central booking at the Manhattan Detention Complex in Lower Manhattan.
Duncan was reportedly working on a motorcycle in his garage with a friend when he was approached by two men who demanded the bike.
Police say he refused and a gunman opened fire, hitting Duncan once in the head. He pronounced dead at Brookdale Hospital.
Officer Duncan's service weapon was recovered at the scene. It appears Duncan returned fire before collapsing, and corrections officials say that may have saved his friend's life.
Police say a gun, possibly belonging to one of the suspects, was also recovered at the Dunkin Donuts.
The corrections union is calling Duncan an exemplary officer. One friend said he was beloved in the neighborhood and loved his motorcycles.
Duncan is survived by a teenage son and daughter. He lived at his house with his mother and sister.
