I heard that you can copy the articles few times let's say about 5 times and google doesn't bother to penalize you. However, what about whole sites? If I create 10 topic-related sites with similar content(duplicate in 50%), how google treat my sites? In fact topic-related sites should have similar content. What's more if some people copy the text from my site and put it on their sites who will be pnalized? How google can chcek who copied the content, it's impossible isn't it?
There is no explicit "duplicate content penalty" in Google. What happens is that when Google determines that two or more documents contain substantially identical information, it selects the best (ie. "canonical") version and de-indexes the duplicates. But if an important page on your site is deindexed in this process by Google, it can certainly damage your rankings by impairing the flow of PageRank through your site, but the major consequence is usually limited to having a page that is essentially invisible to searchers.
If you put duplicate content on 10 sites, there is a possibility of being banned from Adsense. It says in the webmaster guidelines that duplicate content is not acceptable. Don't do it.
I've seen pages marked as DUPLICATE before, but to the exterme state of website being banned also. Thus, as above it seems u need to figure out the level of risk ur willing to take as to the outcome. For myself, i make an effort to edit each new article to be as different as possible from the source - Paragraph switiching, work exchange and more ... I do belive its worth the effort.
"What's more if some people copy the text from my site and put it on their sites who will be pnalized?" I guess the site which crawl first will get the credit for the text.
As time goes on the rules towards duplicate content is become more strict. The best policy for the webmaster is to always work with unique, well written content. You may want to read about LSI and dupe penalties. H