In Charles Dickens's A Tale of Two Cities, there is often a duality to the characters. And, regarding the theme of Duty vs. Desire, there seems again some duality.
As the former servant of Dr. Manette, Ernest Defarge rescues the prisoner freed from the Bastille by the revolutionaries and places him in an apartment behind his wine shop. Out of concern for the doctor, Defarge notifies Tellson's Bank, which, then, summons Mr. Lorry to France. Yet, while he shelters the damaged prisoner, Defarge is not above displaying him to the Jacques as a victim or tyranny in order to promote the cause of hatred against the aristocracy. He also forgoes his affection for his former master when, after asking his wife, Therese Defarge to not punish his daughter and child as she seeks revenge against Charles Darnay, ne Evremonde, the son of the man responsible for the death of her family. For, when Madame Defarge refuses his request, he acquiesces and goes along with her design of revenge.
Unlike Defarge, the dissipated Sydney Carton swears devotion to Lucie Manette in his platonic love for her. When Lucie feels the threat of fate in the Chapter entitled "Echoing Footsteps," Carton promises her that he will do everything in his power to protect her and her family. Later in the narrative, after Darnay is taken prisoner and sentenced to death, Carton again uses his likeness to Darnay--as well as his damaging information against the spy Basard--to gain entry to Darnay's cell and switch places with Lucie's husband. In his absolute devotion to Lucie's happiness and security, Carton offers himself as sacrificial victim to the Revolution. Yet, in so doing, he does attain a certain redemption of himself as he contemplates the "far, far better thing" he does that will leave him with a good name, a name that can be perpetuated in the Darnay children.