Not bad is the only double negative I can think of that is acceptable in the English language. Despite the grammatical problems, this is a very useful phrase, particularly when being asked “How are you?”
I find this an incredibly bizarre phrase. It is simultaneously the most personal, intimate question and the most banal and bog standard one. How you are is a very complex question and covers all parts of a person’s life. It’s usage as an introductory statement is baffling. You may as well just say “Tell me everything”.
From now on, can we just agree to say what we mean? If you want to know everything and will listen, rather than interjecting with your own stories and anecdotes, please continue as you are. But otherwise, if you just want to hear me say I’m alright so you can continue to tell me what you did this week, then please just say something that makes this clear. How about “What’ve you been up to”, as this avoids anything about my feelings.
If you continue to use unclear sentences, then I will respond with an equally ambiguous answer and use poor grammar to boot.