It happens to the best of us, and more often than you'd ever suspect.
Especially with the advent of content scraping 'spiders' that gather entire websites and blogs via RSS feed, then republish the content elsewhere....
This tool is also great if you're purchasing content - to check to see if it's already been published elsewhere, or worse - stolen from a competitor's site (which has happened to at least one member here that I know of).
This is a great reminder thanks for posting!