Many copies of the original Duke Nukem call it Duke Nukum. Here's the reason why, according to 3DRealms.com.
"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."