I am what I look like. Add a letter and I’m not there or I am nothing or I am over. Some chlorine makes me a match, but aluminum would make me unique. What am I?

Riddle: I am what I look like. Add a letter and I’m not there or I am nothing or I am over. Some chlorine makes me a match, but aluminum would make me unique. What am I?

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One (gone, none, done, clone, alone).