IMO, this "paradox" is easily explained. The man that thinks this, is still being controlled by destiny. The thing is, him even THINKING this is destiny (or future, if you will). If he chooses that going to the doctor will solve his problem, because destiny isn't possible, then that was his destiny all along. It is not as simple as anything that can be told to you, as such palm readers, fortune tellers, etc. can't be real. This fact doesn't change the reality that destiny is what's going to happen, clear and simple. Nothing can change something as basic, and inalianable as destiny, because attempting to change it basically IS one's destiny, if they so choose to go that route.