When Apogee, the makers of Duke Nukem, originally conceived their character, the original spelling of Duke Nukem was the same as it is today. They soon realized, however, that someone else had come up with a character with the same name and spelling: Captain Planet's own Duke Nukem
vs. 
Since their Duke Nukem came second, Apogee believed they had to change the name, which they did, temporarily to Duke Nukum. However, they later discovered that Turner had never actually trademarked the name of their Duke Nukem, so Apogee's Duke got the name they originally intended, which is the name that survived
http://www.3drealms.com/faq/apfaq72.txt (search for Nukum in the txt file)
http://en.wikipedia.org/wiki/Dr._Blight#Duke_Nukem
Monster Hunter: pissing me off since 2010.
