Not having conducted enough proper research, the only mainstream musician I can name who had a DUO in his keyboard arsenal is founding Chicago pianist/singer/songwriter Robert Lamm. He probably got hold of his around 1978. The chronology of his arsenal leading up to then included a Hammond B3 organ, a Wurlitzer e piano that was succeeded with a Pianet T, a Steinway concert grand, then the Pianet being succeeded by a Clavinet D6 and a Rhodes Suitcase piano. (Robert also bought a Minimoog synth in 1974 and a Yamaha CS-80 I’m ‘78.) I thought it might’ve been slightly redundant for him to sandwich the DUO between his Rhodes and Minimoog, but I think I understand a lot better why he chose to do that from watching this video.