Question: How do you get someone to break the habit of saying double negatives? 

Answer: By example.  It is rude to correct adults' grammar.  So you set an example.  When you talk to them, use correct grammar in your speaking and hopefully it will rub off on them.  Don't be too obvious about what you are doing.  Don't offend anyone!  Truthfully there isn't much you can do about other people's actions.  Just set a good example.  You ain't got nothing to lose.  LOL  ;-)
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