I think it filters dupes within a certain time frame. You don't want important front-page news at a site being considered a dupe because something was big on the front page a year ago.

Gotcha. In this particular case though, it does feel like every few months someone will remind us there is this great tool some of us have been already using for 2 years now:)

FWIW, I hadn't heard of it before.

(actually that's a lie, I saw it mentioned in the CSV challenge thread. But that was just a few days ago, so the point still stands)