1. Check the type of duplicate content you have
2. Try to remove it from the other domains
The second reason is because most web masters are not the ones copying your work. Usually it is the writers they employ that have stolen your content. There are ways of getting around plagiarism detectors. If you go in all angry then the web master will probably react with anger at both you and the writer they employed.
3. Fix any bugs that may be causing such a problem
You should do bug check on a regular occasion. They often appear through many page refreshes and updates. After a while you end up accidentally creating bugs and there are quite a few that end up looking like duplicate content.
4. Use "noindex, follow" to stop content being indexed
5. Be wary of your boilerplate content
A lot of people will put their logo on every page and follow it with their tag line. If you are planning on doing this then write your tag line on the home page or the about us page and then every time it appears make sure it is within an image. If you put your tag line into an image and then place it besides or under your logo then nobody will know the difference (it should look the same). However, the search engine cannot read the tag line if it is on an image so you will avoid the duplicate content penalty.
Item descriptions are another place where content is duplicated. Firstly, do not ever use the manufacturer’s descriptions because they are already online. Secondly, you either need to write a whole new description for each item or you need to find a way of getting similar items on one page.
Amazon and EBay have gotten around this by allowing people to click drop down menus to pick size, color, shape, etc. It allows the seller to put all of the details onto one page and sell directly from there. It means they do not have to write out hundreds of different web page content snippets. It saves time and is still search engine friendly where Google and the Bing/Yahoo search engines are concerned.