This happened more often than we realize, either for financial preservation (for the rich) or because in small isolated communities there were a limited number of available partners.
In my husband's tree, two of his great-great-grandfather's married sisters and another great-great-grandfather married a first cousin once removed of those sisters. The a son of one sister married the daughter of the other sister (a first cousin marriage). This son and daughter then had a son that married the granddaughter of the cousin (i.e. he married his 3rd cousin once removed).
Since my husband's family stayed in a relatively small area, sometimes it looks more like a plate of spaghetti than a tree!