Rivka's post on the "You keep using that word" thread prompted me to look up duel. I assumed it was formed from the Latin word for two, duo, just as dual is. Turns out it's actually related to bellum, meaning "war," the word that gives us bellicose and belligerent. In Old Latin the root was duellum, but the sound /dw/ often became /b/ in Classical Latin. Another example is the Old Latin prefix dui- becoming the Classical Latin bi- (as in bicycle, bicuspid, and bisexual).
Duel somehow survived this sound change and, under the influence of the word duo, came to mean "war between two people" after being borrowed into English. So it seems people have been confusing and conflating these words for 500 years or more.