I'm sure it's happened to most authors, but I had the terrible realisation that my story has already been written, even if it's in a different form. I was watching TV the other day when a movie trailer came on... and there was my novel.
I don't know if I've ever mentioned what my current novel is about, but it's a sci-fi YA about a fifteen year old girl who contracted a disease two years before the novel begins. Unless a cure is found, it will kill her within the year. If you watch the trailer, there are similarities.
Of course, a few days later, I've calmed down enough to see that there are more differences than similarities. Yes, the characters in the movie contract a mysterious disease that kills quickly. But it's more quickly than in mine. It also hits a lot more people- the disease in my novel is extremely rare. I also only tell the story from a patient's point of view- I never go into the minds of doctors or parents or friends. Still, it freaked me out a bit.
I guess there are no original ideas... and I wouldn't stop writing my novel either way.