I think it's safe to say that it happens to the best of us (I am writing this post partly because of my discovery of what a Death Note is). I've actually had many ideas before they were invented by others, one of them being a take-home box for soup.
My suggestions? Get a new idea. Modify yours. Get there first. The second one is something I often do--take a writing prompt and modify the heck out of it, so it doesn't look remotely like the original. But if a writer is really so clever, he/she can come up with 25 more ideas. Even if the subject matter is similar, I try to differ the circumstances.
I'll try to write longer posts. Maybe I'll make it a goal. ~ I'm still put off that I missed an eclipse. ~ Shortest random notes ever, is it?