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 NukemDuke 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.
Doom's protagonist (Doom Marine but often called Doomguy by fans, or just "Marine") represents the player's characters of Doom and its sequels.