It happens to all writers. If it hasn't happened to you, it will. If it has - don't worry, it's not a conspiracy. And chances are, there's no real reason to get all in a huff about it or try to figure out who to sue. Just suck it up, and move on. I'm talking about that fateful day, when your prized idea appears in the trades as a sale that's being fast-tracked for production. Hopefully you're not completing the final pages of your masterpiece when that happens, because chances are, especially if you're not repped by an agent, manager, or lawyer yet, your script just bought itself an open-ended ticket to the back burner.
The above happened to me this week - twice, actually. I have a scrip that I've been toying with for a while now (the "Roman Army spec" of last year's Writing Weeks). To be fair, this script was already put in the proverbial drawer at my manager's suggestion. Before he suggested we sit on it for a while - mainly, some very big concepts that, without source material such as a comic book to back them up, can be hard for a big studio to be very confident in - other movies came out that made me start to worry. Clash of the Titans, Centurion, and a half-dozen other Roman/Greek movies flooded the market in a short period. The success of the pictures aside, there was a lot out there, and my project began to run the risk of redundancy.
This week, though, two more nails were driven into the coffin. One project had a title similar enough to mine to make me cringe. Swap out one word, and they were the same. That was a small issue compared to a sale recorded later that same afternoon - a project that was in concept quite similar. It wasn't identical, but the combination of a similar idea and nearly identical title being used by two projects in one day made me realize that the drawer-time for my Roman Army spec just got extended.
What if I'm in a pitch or something later on, and I get the sense that the producer I'm meeting with is searching for what I have? Sure, I'll pitch it - why not. Can I swap out Roman for some other historical civilization? Yeah, probably. And for the scrip to have any life now, I might have to. Hmm... Mongols, anyone? Anyway, such is the fate of an aspiring writer. This happens, people. If it does, my guess is that no one stole your idea. You'd need more than the notion of a couple being haunted by an evil spirit to claim plagiarism against Paranormal Activity. Get into the details - not just of your idea and how the film compares, but of who you told it to and the people involved in the production - and you might have more to go on. For the most part, though, these experiences can be considered (unwanted) exercises in developing the thick skin necessary to try and make it in the industry.
Should you automatically delete your script because something similar comes out? No. Definitely not. But, at that point, you need to be realistic. If something is the start of a trend, you might have time to get on board. Maybe. On the other hand, if someone else, someone with clout and credits, is doing what you want to, it's time to weigh your options. The spec you use to break into the industry should seem fresh (these days, the fresher the idea or approach, the better for new talent). If it's way too similar to something already in the works, many producers will stay away from it. They might treat it as a sample - never a bad thing - and you'll get other work from it; but that specific project will gather dust for a while. On the other hand, if you don't want to just aim for a sample, and you have other ideas and aren't too far into that particular script, it might behoove you to move onto something else, one of your other ideas that isn't as close to something that just sold. You'll save yourself a lot of time and grief, and chances are greater that you'll impress someone with you ingenuity.
Of course, take all of that with a grain of salt. Take it with a tablespoon of the stuff. It's all about time management, and if you think that the window for an idea just closed, it might be time to see what others are still open. And if you don't have any other ideas, then by all means, finish that one, and make it as solid as you can. The last thing you want to do is let one similar idea stop you from writing all together. If that were the case, we wouldn't have had any new stories for centuries. Then where would we be?