Yes, a doctor was often on hand but would not view the duel itself. In the generally unlikely event that the matter ended up in court, it allowed them to testify that they had not seen what occurred. That said, it was usually not the case they they would simply turn around, but more commonly they would remain a discrete distance away so that they could be fetched. Contrary to the line in Hamilton, however, this was also the case with Dr. Hosack. He was out of sight, and it was the boatmen who had remained close, and turned around for 'deniability'. I expand on this a bit here in our Hamilton AMA.