With respect, don't confuse translation with meaning. In my example, mei you ren = nobody and bu zhidao = doesn't know, therefore the MEANING may be the same as renren dou zhidao, (everybody knows), but that is not the translation.

In much the same way as 'wo feichang xi huan' means 'I like very much', yet translates as 'I very like', which is considered to be bad English.

Correct me if I am wrong, but I thought all this talk about double negatives was to highlight correct and incorrect grammar use.

Mei you ren bu zhi dao is not grammatically wrong, just as 'she's not unattractive' is not grammatically wrong, yet technically, they are both double negatives.

but better to say 'ren ren dou zhidao.'

Why better? I told you that my NCS boss uses this expression simply for emphasis, so why do you say that it's 'better' to use 'ren ren dou zhi dao'?
Are you also saying that it's 'better' to use the phrase 'She is pretty', rather than 'She's not unattractive'? If so, why?