Last year, I had a client contact me regarding her articles showing up on someone else's site. She was confused as to whether her security was breached somehow with her site, etc. and was very frantic to fix it.
Congratulations, you've been robbed… and strangely, it wasn't a competitor, it was someone who had a 'purely' affiliate based income site who was stealing from many different authors in order to drive traffic to his site in order to sell from his affiliate links.
Before I go on, I saw this excellent article that you might want to read regarding this issue.
I was in that position several years ago. I have some WordPress 'how-to' articles on my site and a scraper grabbed them (videos and all) and put it on a 'Wordpress How-To' site that was pretty much 100% stolen articles embedded in an affiliate link laden site. I found it because my dashboard showed 'incoming links' to my site from theirs.
I posted a 'comment' on his site, demanding he remove my article or give me credit (no shocker the post went into 'moderation' never to be seen) and emailed the 'WHOIS' contact. After a day or so, I went on WordPress.org and published a link to my article and then to his article (he didn't even take out my example link that linked back to my site, LOL!) and in all innocence asked for help. Well, the internet hath no fury like this type of thing happening. Apparently according to the responses I got on WordPress.org, his entire site was scraped articles.
I was THEN (within hours of publishing the complaint) contacted by the site owner who claimed he paid someone to build traffic and removed the article HOWEVER he was being bombarded with angry comments from WordPress.org… could I please remove my complaint (or the link). Well, guess what? WordPress.org reserves the right to allow editing and removal of posts… after a certain point, I was not able to remove it and he ended up taking his site down (probably moved it). I don't know if he would have bothered responding if I hadn't, but I really only posted to get advice.
Don't you love it when you are around to witness Karma in action… it makes you want to take the day off!