Matthew Turpin began his career as a celebrity double after being mobbed by tourists who mistook him for William during a family visit to Windsor. He funded his university degree with his appearances and featured on Blue Peter and Celebrity Stars in Their Eyes.
He died in his sleep at his home in the Oxfordshire village of Heathfield, an inquest heard.
Tests revealed symptoms of cardiomyopathy- which can cause sudden death.
The inquest in Oxford recorded a verdict of death by natural causes. Matthew's father Patrick Turpin, 59, said: "With this illness you don't know if you have got it-and you can go to sleep and not wake up.
"How can anybody come to terms with that?"