In this comment, Rethliopuks mentioned something I'd never really connected in my head before.
[Negative concord] is standard in plenty of languages around the world, incl[uding] most Romance languages, Afrikaans, Greek (Ancient or Modern), nearly all Baltic and Slavic languages, Indo-Iranian languages iirc, Uralic languages, Turkish, etc.
Greek is a language that's famously gone through Jespersen's Cycle more than once, with clear evidence of all the different stages. But it hadn't occurred to me that all those stages had negative concord too. Similarly, the famous example of French ne pas comes from a negative concord language.
Do we ever see Jespersen's Cycle happening in a language without this concord? If so, does the process differ in any significant way? The middle stage of Jespersen's Cycle seems to require multiple negative markers making a net negative, but the Cycle is also talked about as a sort of universal, while negative concord isn't.
P.S. I'm not sure if all of my terminology is standard.
To my understanding, in a negative concord language, multiple negatives make each other stronger, and one element of the sentence being marked negative makes the rest be marked negative too, if possible. For example, Yiddish ikh es an epl "I'm eating an apple", but ikh es nit ken epl
I eat not no apple "I'm not eating an apple".
In a positive concord language, either you can only have one negative per phrase with everything else being positive (English "I haven't ever done it", *"I haven't never done it"). Alternately, multiple negatives can make a positive: Latin non numquam ēgī
not never I.have.done.it = "I've done it at some point".