I have many cases in my ancestors in Brittany, that was indeed if not a duty, at least a frequent thing, both ways.
The most recent one is my maternal great-grandfather Désiré-Marie Lhostis who married the elder sister of his wife after her death at 23. This was really a headache for us children to understand how the cousins fit in the family.
The other way round, a brother of my paternal great-grandfather, Pierre-Marie Vatant, married the widow of his brother, mother of nine. Just the beginning of a long story, see the profile, he married four times ...
And several more. A very frequent and similar pattern in this same rural society was the marriage of two brothers with two sisters. That was the case of my paternal grand-parents (who were neighbours and 4th/5th cousins, and many more above them). Sometimes the double marriage was celebrated the same day.