I (RichyP) took part in an interesting discussion over on the searchenginewatch forums the other day:

http://forums.searchenginewatch.com/showthread.php?p=94871Â

A web site owner was experiencing a ranking with Google that was always 31st position. It seemed whatever the search term the site always came in on page 4 of Google results. The main domain had a PR of 6 so I thought this was strange that they didn’t rank higher. Well I had a little look into this for them and discovered that they had multiple domains all pointing to the same content.  It therefore appears as though Google has penalised them via some form of duplicate content filter.

Never heard of the magic 31st position or page 4 of Google results though but an important lesson on not to serve up the same content under the banner of multiple domains.