I must rest, Maester Cressen told himself. I must have all my strength come dark. My hands must not shake, nor my courage flag. It is a dreadful thing I do, yet it must be done. If there are gods, surely they will forgive me. (1.Prologue.133)
Cressen's technical duty is to serve his lord and the realm as they need him. But his love for Stannis and the belief that Melisandre has an evil influence on him have caused him to forego his duty. Or perhaps he believes killing Melisandre is his true duty?
Tyrion was a little drunk, and very tired. "Tell me, Bronn. If I told you to kill a babe… an infant girl, say, still at her mother's breast… would you do it? Without question?"
"Without question? No." The sellsword rubbed thumb and forefinger together. "I'd ask how much." (9.Tyrion.144-145)
Bronn's duty is to Bronn. More specifically, Bronn's duty is to his bank account. Lords? Honor? Morals? They just get in the way.
Once the maesters in their Citadel had proclaimed the first of autumn, wise men put away a portion of each harvest… though how large a portion was a matter that seemed to require much talk. Lady Hornwood was storing a fifth of her harvest. At Maester Luwin's suggestion, she vowed to increase that to a quarter. (17.Bran.61)
Think it's just peachy being the lord? Maybe it is if you like to micromanage. Bran's duty as Lord of Winterfell is to manage his vassals so they can meet the population's needs, including food and protection.
Fate drives me south and south again, Catelyn thought as she sipped the astringent tea, when it is north I should be going, north to home. She had written to Bran and Rickon, that last night at Riverrun. I do not forget you, my sweet ones, you must believe that. It is only that your brother needs me more. (23.Catelyn.21)
Catelyn's duty is to family, but in this novel, that duty is pulling her in a bunch of directions at once. On the one hand, she wants to see Sansa and Arya rescued, but on the other, she wants to be with Bran and Rickon during this difficult time. And on the third hand, she wants to be with her father when he dies. And that's a lot of hands.
"Why should men fight and die for you?"
"I am their lawful prince," Theon said stiffly.
"By the laws of the green lands, you might be. But we make our own laws here, or have you forgotten?" (25.Theon.174-176)
Theon believes an ironborn's duty is to serve him because he is thinking about duty in the Winterfell sense—but on the Iron Islands, you have to earn the right for people to die for you.
"I have no quarrel with Renly, should he prove dutiful. I am his elder, and his king. I want only what is mine by rights. Renly owes me loyalty and obedience. I mean to have it. From him, and from these other lords." (32.Catelyn.28)
Actually, Stannis kind of has the right of it—according to Seven Kingdoms tradition, the younger brother does have a duty to the older brother. But Renly's argument is that Stannis would make an awful king. So isn't Renly's duty really to the realm and not his brother? Again, different obligations compete with each other.
Knights are sworn to defend the weak, protect women, and fight for the right, but none of them did a thing. Only Ser Dontos had tried to help, and he was no longer a knight, no more than the Imp was, nor the Hound… the Hound hated knights… I hate them too, Sansa thought. They are no true knights, not one of them. (33.Sansa.52)
Sansa thought a knight's function was to do all the stuff mentioned above, but the knights of the Kingsguard feel their duty is to do what the king says. All that other stuff is, ah, just the small print.
"The greater part of his foot remains at Bitterbridge." Varys abandoned the brazier to take his seat at the table. "Most of the lords who rode with Lord Renly to Storm's End have gone over banner-and-blade to Stannis, with all their chivalry." (37.Tyrion.16)
So much for their honor bound duty to Renly's cause… Guess they figured their duty was transferable.
"So many vows… they make you swear and swear. Defend the king. Obey the king. Keep his secrets. Do his bidding. Your life for his. But obey your father. Love your sister. Protect the innocent. Defend the weak. Respect the gods. Obey the laws. It's too much. No matter what you do, you're forsaking one vow or the other." [Jaime] took a healthy swallow of wine and closed his eyes for an instant, leaning his head back against the patch of nitre on the wall. (56.Catelyn.136)
And Jaime nails it. The crux of duty in A Clash of Kings is that it's too much. We see this with Catelyn, Renly, and a bunch of other characters. If all duty is created equal, then how do you choose? And if it's not created equal, then how do you know?
The light was already fading in Qhorin's eyes. "… sharp," he said, lifting his maimed fingers. Then his hand fell, and he was gone.
He knew, [Jon] thought numbly. He knew what they would ask of me. He thought of Samwell Tarly then, of Grenn and Dolorous Edd, of Pyp and Toad back at Castle Black. Had he lost them all, as he had lost Bran and Rickon and Robb? Who was he now? What was he? (69.Jon.101-102)
Jon thought he had this duty thing down: Be honorable, defeat your enemy, and follow orders. But orders have forced him to kill a fellow brother of the Night's Watch. How does that fit into the duty puzzle? Ugh.