With me or against me? Using dual and duel

For today's phonics fun, I'm going to tackle the semi-homophone pair, dual and duel. Most pronounce the words similarly, though one of the pair might have two syllables (dewl; DEW-ul). There may be significant variation here depending on your dialect. The two are most often confused in written contexts, because they sound nearly alike and are spelled nearly alike.
Their meanings, however, are nearly antonyms. Nearly because they aren't the same part of speech. The A version is an adjective, th...