Once you publish an unique article there is a probability to duplicate your content. You can simply use the duplicate content detecting method to make sure that your content is safe. It is important to do a random check time to time. If you find out that someone duplicate your article without your knowledge what can your do as a webmaster minimise the negative impact on your site.
Request to Remove Copied Content from the Webmaster
First thing first. Try to contact the webmaster or the owner of the website and send a friendly request to remove your content from their site or to give appropriate credit to you for the content. Most of the time this method works and do not forget to mention your second step in any case they did not response to you.
Report A Website as Spam
If you did not get a satisfying response from the website you have to move to the next step. Simply report the website as a spam website using webmaster tools. Not only duplicate content but you can report the followings to make the Google better place.
Hidden text or links
Misleading or repeated words
Page does not match Google's description
Cloaked page
Deceptive redirects
Doorway pages
Duplicate site or pages
Other Ways to Report Spam Websites or Splogs
The Anti Spam Extension For Chrome
Google Webmaster Tools
Flag Spam Blogs using Blogger
Use the Adsense Anti Spam Report
On the same time you should make sure that you are not copying someone else hard work without proper permission and this will definitely affect your website ranking, indexing really badly. Make the Internet better place.