I can tell you how it can be an accident. I think I have five duplicates in mine because I was using some software to upload the pictures and, for some reason, it uploaded some of them twice when the site when down and I had to redo it. I just haven't had a chance to go through, find them and delete them yet. Of course, not all of mine have tags either. However, if someone is just uploading the same ones over and over and over and over again, that certainly isn't right.
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http://www.flickr.com/photos/narilia