be one's duty to

My dear child," said Mrs. Light, "it may be one's duty to pose.

That is, it would be one's duty to make the great moral sacrifice of speaking an untruth for the sake of saving the mother.

And with her, Nancy, dressed for dinner too, because Mrs. Winters feels it to be one's duty to oneself to dress for dinner always, no matter how much one's guests may wish to relax—Nancy as much out of place in the apartment whose very cushions seem to smell of that modern old-maidishness that takes itself for superior feminist virtue as a crocus would be in an exhibition of wool flowers—a Nancy who doesn't talk much and has faint blue stains under her eyes.

A critic was saying that it would be one's duty to do a pretty bit of slashing.