Well there is probably a list of names that is used to generate random chieftains, so you are bound to run into same name chieftain sooner or later. Ive dont at least 40 “kill orcs” missions since the big update, and i dont remember running into a single chieftain. Those probably start appearing at higher renown or something.
Nah, you have to go hunting for them in the wilderness. They live in what are practically orcish cities.