Duplicate Content Penalty

Can anyone explain what consitutes as duplicate content? How do I know if my sites are being filtered?

I have a network of seven similarly themed web sites but each with a unique subject.

I think some of my sites have been penalized by th duplicate content filter and I am not sure how to recover from this. The reason I think this is that some of my sites rank very well while others don't show up at all in the search results but they are in the index. All sites have page rank of 5 or higher.

The sites used to be cross linked but I removed all links to the other sites several months ago. Is it only content that Google looks at or is it also page strucure? Should I significantly change the content on those sites that are not appearing? I guess the larger question is what does a site have to do to recover from the duplicate content filter?

I did some additional research and found that several of my sites are on the same class-c block. I have read this can be a problem.

I have also read that if Google thinks content has been duplicated, it will allow the first instance of the content and not show all of the other duplicate instances. This fits with what I have seen with my sites.

I am unsure how to fix the situation however.

Thanks in advance for any assistance anyone can offer.