this link to 'Inca roads' of Frank Zappa & the Mothers of Invention's 1975 album 'One size fits all'. 'Inca roads' says it all where George Duke's singing and playing is concerned. Easily Frank Zappa's best song ever, one of the most interesting and diverse songs ever and without a doubt the best George Duke ever recorded. Listen and be stunned by the time changes, the virtuosity, the instrumentation and singing. For those seeking a little more technical insight, I can refer you to Wikipedia, that has a lemma dedicated to 'Inca roads'.
P.s. There is a review of 'One size fits all' on this blog here.