i have a stupid question.but : what is the duplicate content ? duplicate content count every site or just one site ?
does it hurt site's rank or google friendly ?

Duplicate content is content that appears on the Internet in more than one place (URL). This is a problem because when there are more than one piece of identical content on the Internet, it is difficult for search engines to decide which version is more relevant to a given search query. To provide the best search experience, search engines will rarely show multiple, duplicate pieces of content and thus, are forced to choose which version is most likely to be the original (or best).

nah.. i've just duplicated the seomoz content to bhw.

duplicate content is what google hates the most. Never use it if you want to have good results. The rest was described by wpbacklinks