How can scholars of Jarh and ta'adil be certain about the identity of a narrator?

Usually if you check a narrator chain -as a layman- you will find something like: > Abu Yusuf has narrated from 'Abdullah ibn Sa'id who heard Muhammad ibn al-Qassim who heard abu Hurrairah ...This is just an example from my own imagination if you found a similar narrator chain by coincidence it was unintended I guess that in the first hijri century there were much more than one single person named Muhammad ibn al-Qassim (note this is just an example) and we could go on this way with any other narrator in the chain unless the muhaddith adds some extra information that may help better identifying a narrator. I wonder how and to what extent the scholars of al-Jarh wa a-Ta'adil actually are certain that Abu Yusuf (for example refers to the student of Abu Hanifa) or 'Abdullah ibn Sa'id (for example refers to "the muhaddith from Yemen" not the one from "Khorassan"). I'd guess usually a Muhaddith referring to Abu Yusuf the student of abu Hanifa would have added the attribute "the qadi" to be more precise. My question is how can scholars of al-Jarh wa a-Ta'adil identify a narrator and what would they do if there could be more than one option according the given facts (methodology)?