Why did so many '80s band names consist of the same word twice?

Yeah, but it evokes that same primal/childish or child-like association. As for the "real" origin of "dada," there are contradictory accounts of how the word was adopted (surprise, surprise), but the Dadaists seemed to relish the potentially childish meanings (e.g., rocking horse).

Well, it can act as an intensifier in other languages too - "very very good" is better than "very good"

Good point.