Seeing double? Not quite! Dual is two, or double, but a duel is a fight. If you're getting sick of your fair-weather friend's dual personality, perhaps you should throw down your glove and challenge him to a duel at high noon.

Dual, which dates from the early 1600s, is borrowed from the Latin dualis, from duo, for two. Dual still means double or two similar parts:

The ego becomes dual, one part active, the other watching and judging. (Max Simon Nordau)

Last night's game had kind of a dual purpose — playing Oregon and sending out this version of Husky Stadium. (Seattle Times)

Duel once referred to an arranged, formal contest to the death between two people, preferably men with handlebar mustaches. These formal contests are no longer practiced, so duel has broadened to mean any contest between two people, teams, or even ideas:

In early days a properly regulated duel was an ordeal showing the judgment of heaven. (Max Simon Nordau)

In a savage duel between the regime and Islamist guerrillas, entire villages were wiped out. (Business Week)

There are dueling pension propositions on Tuesday's ballot. (Washington Post)

Save dual for double, for things that come in pairs, like your hot/cold friend with the dual personality. Use duel for an old timey sword fight, or any modern version of it. Handlebar mustache not required.