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

Question

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'.

Answer 1

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."