I do. I hate that.
After I wrote the superhero book, out came The Incredibles, Sky High, and Heroes.
Last year a book came out with the same subject matter as a book I wrote a few years back.
Then last week I found out there was a middle grade book with the same fantasy theme as one I just started writing!!!
The thing is, even though it's kind of annoying, it doesn't matter. Give two people or five or ten for that matter the same character, same setting, same plot line even and you're still going to get ten different manuscripts. No one will write the same story.
So next time you have that great next big idea, then find out someone's already writing it, don't get discouraged, don't stop, keep going. That other person and you can never write the exact same book. And hey, yours may be better. :)