It’s not true. Harry is Charles’s son. There is quite a resemblance with Prince Philip, and as Harry’s getting older you can see more of Charles too. The red hair comes from the Spencers. The guy Harry looks like (James Hewitt) didn’t sleep with Diana until after Harry was born. I am 100% sure Harry is the biological son of Charles.
Hewitt looking like Harry is just a coincidence. Harry mostly looks like Philip and Charles Spencer.