If you rely solely on Google generic search engine result this can be a huge problem. But again the boundary at which one content can be considered duplicated is a vague in itself.

I have seen many duplicated content rank well by just inserting 100 words intro and 50 words comment in the middle and the end of the article.

So duplicated content is not something that you should take full concern. There are many ways to utilize duplicated content. Giving free report tactic...