I don't know if there's a word for a nightmare daydream, but have you ever had one where you're sitting in a movie theater watching the previews and suddenly you see a trailer for a movie that somehow is exactly like the book you're working on? No? Just me?
I think a lot of writers can be paranoid. It comes with the territory. If you have a great idea in your head, then you would be devastated if someone stole it. Especially for those of us who blog about their writing progress, there's always that little voice in the back of your head worrying about who is going to see your words and what they could do with them.
I was recently browsing books on Amazon and came across a novel that had a few similar elements to my WIP. And I proceeded to freak out. There's that moment where it feels like your whole life is over. You think all of your hard work has been for nothing because somebody has already done it. So what's the point in finishing?
You have to think of it this way: Everything's been done before. And yet, it hasn't. There are so many elements that go into a story that make it unique. Sure, there are only so many basic plot structures, and they've been used countless times. But you've got to factor in all of the details of a story--characters, plot twists, subplot. There's no way that you could accidentally write a story that's 100% been done before. Every element of a story makes it different. Like, if I changed my main character, if I swapped genders, changed to third person, or switched point of view. Each one of those changes would be an entirely different story. If I set it in the rural South instead of New York City. Different story. If my protagonist came from a nuclear family instead of a single parent household. Different story. Do you see how many things impact the story you're writing? And you come up with every single one of them.
As I read more about the book, I realized it really had barely anything in common with my WIP. Every story is unique and exists all on its own. In fact, after finally having some extra money, I bought the book and I'm looking forward to reading it. I love reading books in the genre I'm writing, because it helps inspire me. It's not that I look for ideas in these books, but that I wait for them to trigger ideas of my own.
And I know there's always the paranoia of theft. Sometimes we don't want to display any detail about our stories because we're afraid someone will steal it. But once again, it really comes down to all of those elements. Even if you gave just a basic summary and someone did steal it, what are the odds that they would end up writing the same story as you? They couldn't possibly guess every detail that you have planned. And chances are, if they are stealing, they probably aren't as good a writer as you and they won't be able to pull it off.
So write without fear! Your story is completely yours, and no one could write it quite like you.