"Το νερό είναι σημαντικό για ανθρώπους" is currently marked as incorrect, general strengthening on web. The second article isn't strictly necessary though (or is it?) If we were talking about something there is only one of (eg 'humanity') I could maybe see the need for it, but we're talking about humans in general, right?
Normally, the article wouldn't be necessary, but I must say the sentence looks kind of awkward without it. Even if the speaker refers to humanity, it seems like something is missing here. Ι think it's the fact that it's barely ever used without the article that makes it look strange.