Praenomens in ancient Rome seemed to have been rather limited, especially by the time of the empire. There must've been cases where close friends or cousins and the like would've had the same praenomen. Since close relations adressed each other by their praenomen, how was this handled? Did they then use nicknames instead?
The Romans apparently had nicknames (of a sort) known as agnomen: Thus for instance Publius Cornelius Scipio had the agnomen Africanus from his military victories.
Hence the development of cognomen and the tradition of tria nomina, such as for Gaius Julius Caesar. Tradition has it that Caesar was born with a good head of hair, and thus acquired the cognomen Caesar from the Latin "a caesariē", translated "because of the hair" or, more colloquially, "Hairy".