There is no such thing as a duplicate content penalty on Google. They will NOT downgrade your rankings because of duplicate content.
Watch this Vanessa Fox interview
(back when she was still a Google employee) for more information about (among other things) what they do
have: a duplicate content filter
. Very different animal from a penalty.
You can use CopyScape to check for stolen content. Another method is to simply search for a unique sentence or two from each of your articles.
If your website content has been stolen, that's a copyright issue. You can send the thief a cease & desist letter demanding they take down the content. You can file a DMCA request with their ISP to have the content (or even their entire website) taken down. And you can file DMCA notices with the SE's. They have been known to remove individual pages and even entire sites from their index if the content on the page was stolen.