Name some Hip-Hop duos.

Hip-Hop has seen many duos, apart from rappers who frequently collaborate there are some which form a duo under a separate name some of these are:

Run the Jewels (El-P and Killer Mike)
Mob Depp (Prodigy and Havoc)
Black Starr (Talib Kweli and Mos Def)
Kids See Ghosts (Kanye West and Kid Cudi)
Bad meets Evil (Eminem and Royce da 5'9")
OutKast (Andre 3000 and Big Boi)