The story of "So What"

While we were discussing the convoluted story of the "Rhythm-A-Ning" riff, my worthy constituent Michael 'Jazz Lives' Steinman, purveyor of happiness, said that

certain riffs and variations are "in the air"

which is absolutely true. From outright plagiarism to excessive, but unintentional, "inspiration", these things can happen (I once put together a really pretty set of chords worth of Rachmaninov, so much so that... you get my drift). And it can happen to anyone, even master composer Benny Golson, as he explains to another master composer, Horace Silver, here.

That's all very well, but in American popular music there was ("was", hopefully) laxity when it came to naming the author(s) of a tune. Take Thelonious Monk's "'Round Midnight". How much did Cootie Williams have to do with it, beyond being the first band leader to record it? This is just an example: any unlikely name, from disc-jockey Alan Freed's name on rock'n'roll records, to manager Irving Mills's on Duke Ellington's, it'd be healthy to raise a brow or two.

Another example: Gil Evans only discovered that "Donna Lee" was not Charlie Parker's tune when he asked him about it, to arrange it for Claude Thornhill, and Parker sent him to his  sideman, Miles Davis, who was just 21 at the time.