So what if it's not completely original?

I don't see why it needs to be. Nothing can be truly original, after all. It's all about variations on a theme. The details are what matter, what make the film unique and enjoyable.

Yeah. Psycho was published in 1959. And it dealt with schizophrenia. Does that mean that the theme should have been off-limits for all fiction written afterwards?

Also: I feel that the best storytelling is more about the journey than the destination. Secret Window wraps up rather quickly, but there are about seventy minutes of film before the denouement that you could try and enjoy on their own merit. If an evasive ending were the be-all and end-all of good filmmaking,there would be no reason to own movies or rewatch them. After all, you'd already know what's coming.