I get a lot of duplicate content related questions when webmasters ask about publishing content both on their site AND external sites.
Obviously you want as much traffic and link juice as possible, not to mention you want to publish your best articles at as many places online as possible, but you don’t want to get penalized for it, either.
To set the record straight, there technically is no such thing as a duplicate content penalty on different websites, only on the same site.
For example, if you post the same article on your site in two different places, that’s a blatant example of duplicate content which can hurt you.
There are also examples of unintentional duplicate content on one site such as regular versions of a page plus a stripped down version designated for mobile devices or having printer ready versions of a page.
Basically Google doesn’t care if the exact same content exists in two places on the same site so long as it’s not intentional.
The only issue that arises here is that Google can only index one page but doesn’t know which to choose, and sometimes it becomes the one the webmaster doesn’t want. Use 301 redirects when applicable, as well, to avoid confusion.
Remember to use the canonical tag to distinguish these issues ahead of time.
When it comes to other sites, duplicate content penalization is again largely a myth when it comes to Google itself. Unless you’re reposting the same article on DOZENS and DOZENS of other sites, there generally isn’t anything wrong with reposting content from your site elsewhere online.
Spokespeople from Google have clearly pointed out that they don’t bother with most of these cases, and it’s only when it’s very malicious and intentional.
Some sites such as higher profile article directories have issues with your reposting content from your site to theirs just because they don’t want to republish information which is already available elsewhere.
If you’re feeling uptight about it, then think about editing a bit of your original article before you submit it elsewhere to make it unique again.
You should do this with the bigger article directories in particular who are more likely to check and search for that content before they’ll accept it, but again there is no site duplicate content penalty for doing this.