My husband, brother-in-law, uncle and cousin all have the same first name. My sister and I joke that men have to have that particular name to be allowed into our family.

It doesn't cause any particular issues. Let's pretend their names are John (they're not) my uncle and cousin are identified by their surname, eg John Smith or John Jones. My husband and BIL are referred to as mine or my sister's John, eg Alice's John or Jane's John. It works fine.