In A Tale Of Two Cities, how are the witnesses John Barsad and Robert Cly discredited at Charles Darnay's trial for espionage?

Robert Cly and John Barsad are suspected (and later revealed) to be spies for the French government. This casts doubt on their reliability in their denunciation of Charles Darnay, who is on trial for espionage when he was returning to England from France. He had just renounced his title and inheritance as an aristocrat and planned to make his life in the freedom of England. Barsad’s testimony is centered on his recognition of Darnay on the ferry crossing the English Channel. Sydney Carton, however, passes a note to Stryver, the lawyer defending Charles, on his similar appearance to Charles. Stryver instructs Carton to remove his wig and stand, so the two of them can be compared. The court is struck by the similarity and thus casts doubt on Barsad’s ability to indentify him with any surety. This supplies enough doubt on Barsad’s testimony, and thus Charles Darnay is acquitted.