In January 1779 he sat as a member of the court-martial of Admiral Keppel for the poor performance of the Channel Fleet during the First Battle of Ushant. During the course of the trial Duncan objected several times to stop the prosecutor in irrelevant and in leading questions, or in perversions of answers. The Admiralty was therefore desirous that he should not sit on the court-martial of Sir Hugh Palliser for failure to obey orders during the same battle. The court-martial was set for April. The day before the assembling of the court the admiralty sent down orders for "Monarch" to go to St. Helens. Her crew, however, refused to weigh the anchor until they were paid their advance; and as this could not be done in time, "Monarch" was still in Portsmouth harbour when the signal for the court-martial was made; so that, sorely against the wishes of the admiralty, Duncan sat on this court-martial also.