Why would he use two negative forms in a row? If someone says "I don't do nothing", isn't it technically have the same meaning as "I do anything"?
Good question. Technically, you are correct that the one negative would cancel out the other! However, "I don't do nothing" means "I don't do anything". While it is not proper English, "Nothing" can be slang for "anything"! Ex: I sit at home all day. I don't do nothing until I go to bed! Ex: You want me to start a business? I'm not a businessman. I've never done nothing of the kind.
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