Check out this nice blog post on duplicate content.
Matt says, "typically a whitehat site doesn’t neet to worry about 1-3 versions of an article on their own site."
But Matt cautions about having your articles syndicated in many places on the Internet since Google finds it harder to know if you wrote the article. If you do syndicate the article, be sure to include a link to the original article on your website. This helps Google determine which article to index. Now I see that this was posted on Feb 1, 2008 which causes the concern that maybe it is out of date.
Note what Matt says about buying links and paid links. In a few words, you can do it but Google may pounce on you for it in the future. Just be aware that paid links are not considered good practice by Google.
Here is a nice video of Matt being interviewed on duplicate content:
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