Doesn't everyone do that? My brother has a grandparents name as his middle. I never thought of it as a gendered tradition

Is it a specific cultural thing I wonder

All the men on my dad's side of the family have the same middle name, with just a different first name.

It's apparently not an uncommon tradition in several areas of the world to either have either the first or middle name be shared, and then a unique name for the unmatched one (e.g. John Alan Doe, John James Doe, John Matthew Doe - Alan John Doe, James John Doe, Matthew John Doe).

Possibly. I’ve just heard it more with girls and wouldn’t expect it’s evenly common on relative basis but certainly could be wrong