We've seen many comments here at WW about the value of sharing articles with other sites.
It seems like you might be on that path by chance rather than choice. It's possible that you can make this situation work rather well for you.
The duplicate content issue is important. Offering an abstract of the article, or overview/mini-article to the other sites might be a way around that. They get a brief version of the whole article in return for a link back to you, ie: "read the whole article at...."
Depending on how many articles have been ...umm.... "appropriated", you might even be able to go back to those sites and offer these overviews as a replacement for the appropriated full article.
In the future, perhaps you could prepare the overviews at the same time you write the main article. Then offer the overview as a standard practice. Just put a note at the end of each article you write with a link to the overview and the criteria for using it.
The advantage here is that you can more or less control the wording of the overview version, thus avoiding more of the duplicate content risk.
[It's a bit off topic, but somewhat related that I had a similar issue with regard to images being "borrowed". My concern was that making an fuss about the image use could really harm me in the community I was attempting to reach.
After some discussion here at WW, my solution was to simply label the images with my domain name and not worry about it. Suddenly my domain name was appearing on a lot of websites. My traffic immediately went up significantly.]