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'.