How do the writers here handle the issue of working with an idea that seems as if it probably has already been done?

This drives me bananas. Sometimes I come up with a cool idea, write a story around it, and then suddenly freeze before sending it out because I figure someone has already covered the same ground. As an example, I have an idea for a YouTube-type site that returns results from the future, but I cannot get myself to write it since this seems like an obvious one. I haven't found a story like this yet, but I was wondering what kind of searches people perform, and what websites they may use, to see if a story idea already exists.