There can be parallel development. Or he figured it out on his own when no one told him the companies had been doing it, even for years. No Internet back then to yell out "Hey listening device, how to keep pickups from feeding back?" I thought it was fun touring Les Paul's inventions at a museum where it included his recording equipment and finding that he had used several tape machines to record and play back so he could over dub when no one else had been doing that ... when I had done that same technique as a kid with my younger brother's and my cassette tape recorders running playback while adding the next 'track' playing piano, adding drum beats, and so on. I was probably ten or eleven with a seven or eight year old studio assistant and co-musician. .