There's a bunch of stuff in Duke Nukem that they never used. First is this alternate title screen for episode 3 stored in a file called END.DN3.
Next is this weird animation and a black Duke with a red shirt and blond hair, found in all of the MAN0.DNX files (thanks Frenkel for the pictures).
Finally is this weird bit of text I found in the Duke 3 EXE.
You'll not get away butthead! Butthead, it's buthead!! The spelling of yours is getting better! Finally, Earth is free from Dr. Proton and his Techbot menace.
Copied Art from Duke Nukem 1
Duke "borrowed" graphics from a few other games, as all artists and game designers are lazy. Particularly relevant are crates and barrels, which becuase they are extremely easy to draw and fit in almost any game atmosphere, prove the true work ethic of the designers for Duke Nukem.
Duke vs. Mega Man
Also, Duke borrowed a lot of graphics from a game called Turrican (source).
Duke Nukem Naming Issues
There was a small controversy over the correct name of Duke Nukem. Apogee eventually released this statement clearing up the matter:
There is frequently a great deal of confusion over the correct spelling of one of Apogee's most prized characters, Duke Nukem. The original spelling for the name was "Duke Nukem"; however, during the code fix stage between v1.0 and v2.0 of the original game, Apogee found a character overseas named "Duke Nukem" that was thought to be copyrighted. So, for v2.0 of the original Duke game, the name was changed to "Duke Nukum." Then, during the programming of the next Duke Nukem game (Duke Nukem II), it was discovered that the character they found overseas was not copyrighted after all -- and so the original name was reinstated. "Duke Nukem" is also the spelling used in all successive games that featured the character.
The spellings "Duke Nuke 'em," "Duke Nuke'um," "Duke Nuke'm," "Duke Nuk'em" or other variations are not, and were never, correct. The correct spelling of the game is "Duke Nukem".
There's a character named Duke Nukem in the cartoon Captain Planet, with a voice done by Jeff Goldblum.
Duke Nukem sports a dark red mohawk, his skin has become hard and scaly, and his eyes have developed x-ray vision. He sometimes wears coveralls that bare his arms and part of his chest but his favorite outfit is a Hawaiian shirt that he wears while "catching some rays." "Radiation bathing" is the Duke's version of sunbathing and recharges his radioactive powers.