Can the word "駄目" effectively 'cancel out' a negative?


This is a line from a story:

> Speaker C: どのみち先に言わないと駄目でしょ……。
>
> Some context: A has withheld information from B and has fed B false
> information instead as a joke, A says the correct information to himself
> after B leaves but C hears it and rebukes A for doing that mischievous act,
> saying the above line.

I believe it would translate to something like:

> You should say it beforehand anyway.

But literally I believe it would be:

> Anyway, it must not be [the case] that you don't​ say it beforehand.

Given that my above translations are accurate, does that mean the word 駄目 can
effectively 'cancel out' a negative in the same sentence? In this case 言わない -
don't say, but in context the speaker is implying "you should say it" right?
So that would mean the negative is in actuality 'canceled out'.




言わないと駄目 literally means:

言わない = don't say  
と = if (conjunctive particle)  
駄目 = no good

"No good if you don't say", hence "You should say."

> 「どのみち先に言わ **ないと駄目** でしょ……。」  
>  ( _lit._ ) "Anyway, it will be _no good if_ you _don't_ ​ say it
> beforehand."  
>  → "You _should_ say it beforehand anyway."

