Trouble with reposts, too. Read an article, figured it had been submitted here. Searched for it, didn't find it. Okay, fine. Submitted it, tagged it - next thing that happened was I was taken to the article, which had been submitted a month previously. You should be able to re-tag posts. An article about Google Maps should show up under #technology, not just #learnsomethingamazing.
Thanks. That made me realize that the url search wasn't working. I just fixed it. Drop a url in the search box, and if it was submitted, it should bring it up. However, it only works for the exact url, but so does the post redirect. Maybe I could work on a confirmation page for very similar urls, or, maybe even allow dupes, just with a confirmation page letting you know that a duplicate exists.