Common content is different from duplicate content. It means content that remains the same from page to page on your site.
For example, I used to use a ‘lowerpage.html’ file which had all of my feeds / blogs on it. Since it made up about 75% of the content on the page… AND it was used for every page on my site, I got the boot from Yahoo. Once I removed this ‘common content’, Yahoo re-indexed my site and I am doing well with them.