Duel and dual are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It suggests what word you may want to save time, but quite often, its suggestions couldn’t be more off base and produces humorous results.
Duel (pronounced “dool” or “doo-ell”) is noun. It means a confrontation with deadly weapons by two parties seeking to settle a quarrel. A famous one in American history involved former Treasury Secretary Alexander Hamilton and Vice President Aaron Burr.
It can also mean a feud, a competition, or rivalry between two people or companies.
Dual (pronounced “dew-uhll”) is an adjective. It describes something happening between two people or things, or an object designed for two or more people. For example, some master bathrooms (in houses) feature dual sinks so two people can use their own faucet at one time.
It can also describe an object with two or more uses, for example, a dual purpose wine opener that also opens flip cap bottles.
It can also refer to a split personality, a Jekyll and Hyde type temperament.
The following story uses both words correctly:
Dulce and Demetri wanted to dance with the same girls at the cotillion, so they were badmouthing each other to win the girls’ favors. They were getting increasingly louder about it, talking smack and throwing shade the entire time they danced with each stranger for minuets and reels.
Eventually the music stopped and they agreed to step outside and duel with swords. Demetri would suffer a crushing blow to his leg; his dancing days were done for the season. Dulce won, but at a cost: now he was known as a man with a dual nature: capable of being a gentleman, but when people got in the way of what he wanted, he could be downright vicious.