I’m a great fan of movies. After so many years of watching films I decided to write something. One day I conceived a subject. I developed it in my mind until I decided to write it down. But, unfortunately or fortunately, I saw VANILLA SKY. My main story and character is very much like that film. Even some details are exactly the same. What would you do if you watched a movie that is very close to the story you’re writing?
A philosopher who’s studied up on the nascent field of memetics might argue that people don’t actually think of ideas. The ideas are already out there, competing with each other to get people to think them.
In the case of your movie, there was an idea floating out there about a guy who was experiencing life strangely because, it turned out, he was already dead and dreaming. This idea came to you. Unfortunately, it also came to Cameron Crowe, in the form of the Spanish director Alejandro Amenabar’s movie ABRE LOS OJOS.
Back in college, I kept thinking about doing a movie or TV series about an asteroid headed for Earth. I wasn’t the first person to come across this idea. I opened the trades one day to find that ARMAGEDDON and DEEP IMPACT were suddenly racing into production. I was a little bummed, but reassured to think that at least I was capable of a commercial idea.
If you’re halfway through a script and you see a movie that is almost exactly your story, then you have fair reason to moan and cry and tear your hair out. In general, though, writers who abandon one of their projects because "it was too much like" whatever, were just looking for an easy excuse to stop writing it.