Uhh... this is called syndication and you should be happy about it. This is how your work goes "viral" and that's what you want! If they take the link out and put theirs in then I would contact them, but other than that let it ride! Duplicate content is nothing to worry about as long as it's YOUR work, and it's not published twice on 1 site or 1 IP address.
It's not syndication, it's copyright infringement. The part about it not not hurting you unless it is being published on the same IP address is complete BS.
No, the "theory" of duplicate content is what is B.S. Anyone who knows anything knows you don't get penalized for duplicate content.
Actually, all you have to do is ask them nicely. If they continue to rip of your content then find every social network on the web that they are on and destroy their online presence.
Can you read? While there is no actual "penalty" for duplicate content, your site might not show up in search results for your own content. Call it what you want, another site stealing your content can rank for "YOUR ARTICLE" and your site doesn't show up in results. Google ranks according to site authority, not the first to post the content. Only a moron would think google would return 100 copies of the same article in a keyword search. Now look up the definition of "syndication" so next time you can use the term correctly. NOOBS.