Duty refers to actions you feel you have to take; discipline refers to actions you feel you've chosen to take.
Perhaps this is Western thinking applied to a world that includes views that vary wildly from my thoughts, but I have decided to express them regardless of the consequences.
If you promise someone something, and you truly intend on fulfilling that promise, doing so would be your duty. All of your actions that work toward fulfilling that promise fill you with tension. You see it as an obligation that must be fulfilled which means regardless of whether you are successful or not, you can't get lasting enjoyment out of doing it. After all, the person who you vowed to please is the recipient of the happiness, and the best you can hope for is the false reward of someone being grateful, and thus you will never be happy.
On the other hand, if you buy someone the same thing and give it to them without the condidition of a promise, you did it out of the goodness of your heart. And what is true goodness? Not the chains of duty not the weight of sloth, but the liberty of discipline.
Discipline is not rigid self-flaggelation, nor is it the daily exercise of doing something you don't want to so but just "have to" anyway. It's not blind commitment -- these things apply to work and addiction, the two faces of duty. No, discipline requires choice and the understanding of the consequences of the choices made. Discipline means that there is an active, not automatic, decision made on a regular basis to act or to remain still.
That's not to say duty doesn't have its uses. But duty leads to impatience and dissatisfaction. Discipline is an important step in healing and self-development.
Wow...I just got all new age there...and all typoed out...