I've got a great idea for an open source project. What should I do before starting, to make sure that I'm not duplicating anyone's efforts?

Of course I've googled, but which keywords to use isn't obvious. Is there a site to discuss such ideas?

To address a common answer: My goal is not to be the author of a successful project. It is to make sure that this product exists, and can be improved. (There's already a similar commercial venture, which is of low quality and not being developed.) Thus, it won't help me to have the only project that can be found in this niche. If there's a start out there, I'd like to find it and help.