I'm not big on duty, but I think that people who are would consider this a generally plausible principle:
Conditional Release: If A has a duty to B, B can conditionally release A from that duty by choosing that A act otherwise, rather than fulfill the duty. Then if A acts otherwise and does not fulfill the duty, A has not violated the duty.
Now what happens in the case of a self-regarding duty, where you are both A and B? Then you can choose to act otherwise rather than fulfilling the duty, and when you do it, you haven't violated any duties. So it's pretty hard to violate duties to yourself.
Maybe there are some duties for which Conditional Release doesn't hold. Maybe I have a duty not to kill you, and you can't release me from it. Then you could still get a duty not to commit suicide. And if you can violate duties without making any choices, perhaps by falling asleep at an unfortunate time or forgetting about them, this won't get you off the hook for those duties.
More on Thomson's understanding of philosophy
16 hours ago