Actually i can see why this is perceived as a paradox. For one thing, if destiny did actually exist, then there would be nothing you can do to stop it. Let's say if you were to try and stop something from happening, then you'll be the one to have caused it in the first place. It will just loop back around. Also, for the Whovians here, it's basically a fixed point in time. Theres nothing you can do to stop the future that has already been set into motion.