These are joke units and don't appear in regular play. The only way to get them is place them on a pre-made map or by doing a map.ini edit and enabling them.
Duke Nukem[]
Duke Nukem is a fictional character and main protagonist of the Duke Nukem series of video games. The character first appeared in the 1991 video game Duke Nukem, developed by Apogee Software.
Doomguy[]
Doom's protagonist (Doom Marine but often called Doomguy by fans, or just "Marine") represents the player's characters of Doom and its sequels.