But 'mei you ren bu zhi dao' is generally much stronger

Precisely.

Isn't that WHY we use double negatives in spoken English - to give stronger emphasis? (As in, 'I ain't going nowhere!')

You didn't 'upset' me either. The issue that I have is that the academics who post here will not admit when they are wrong.

The Doc made a statement saying that double negatives don't exist in Chinese.

I was merely pointing out that her statement is incorrect.