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