"Juan Sánchez" literally means "John, son of Sancho". For a few dukes of Gascony, the second name is not the given name of their father, but it is a nickname that they gained over time and that replaced the given name of their father, such as the famous duke Sans III Mitarra, where Mitarra is not the name of his father, but a nickname referring to his origin, probably "Menditarra", with a typical Basque -tarra suffix to express origin.
"Juan Sánchez" literally means "John, son of Sancho". In certain instances for dukes of Gascony, the second part of their name is not their father's given name but a nickname they acquired and that eventually replaced it. The example of Duke Sans III Mitarra illustrates this concept.