I’ve heard that too. Probably a lot of folks named with the local equivalent of “John Johnson.”
I knew a couple in college that had two kids. I believe the wife took the husband’s last name. His name was something like Frer Thorson, and the wife’s name was Inga Thorson.
Their little boy was Thor Frerson, and the daughter was something Ingandaughter (or however one spells daughter in Iceland - dotter?) Pretty confusing.
Of course one first cousin marriage every 5 generations is about equivalent to a 5th cousin marriage every generation. The Pharaohs had dynasties of several hundred years based on sibling marriage. The recessive genes would be filtered out rather quickly by infanticide. The Hawaiian royalty also had sibling marriage. We are all pretty closely related, with our DNA being more than 99% common just by being human. It is likely that some of our visual diversity is due to a different corps of parasites living in/on us, and not so much our DNA variation.
Or Jane Johnsdaughter, as the case may be - Denmark only fairly recently dropped the practice themselves.