I thought this article
did a good job explaining some misconceptions about duplicate content. Here are the bullet points:
- Google wants to serve up unique results and does a great job of picking a version of your content to show if your sites includes duplication. If you don't want to worry about sorting through duplication on your site, you can let us worry about it instead.
- Duplicate content doesn't cause your site to be penalized. If duplicate pages are detected, one version will be returned in the search results to ensure variety for searchers.
- Duplicate content doesn't cause your site to be placed in the supplemental index. Duplication may indirectly influence this however, if links to your pages are split among the various versions, causing lower per-page PageRank.
So it is reassuring to know that your site won't be penalized for duplicate content. And what I found most interesting was the idea that what causes supplemental pages is the splitting of pagerank among too many pages. I'm going to have to give that idea some thought, as I think it might help me better distribute my links to the most important pages of my site.