Should I worry about duplicate content? This is a question I am often asked and the answer is “you should be aware of it and the results it may cause.”
If you are trying to get high rankings on Google for a web page you should make sure the content is unique. When Google finds 2 or more pages with the same content it will select the one it thinks is the original and the other copies can end up in Google’s supplemental index. The means that a copy of your content could end up on page one of Google and your web page disappears completely.
Pages in the supplemental index are crawled less and are considered to be trussed less and probably carry less weight regarding their outbound links. Therefore you will not achieve top rankings if your pages are in this index.
You should be particularly careful if you syndicate articles on your blog and I recommend against this. For example if you post an article on your website and then post the same article to a number of article directories or other websites it is possible one of the copies will rank in Google and your page will be pushed into the Supplemental index.
This is why you will find that people will use article spinners to get hundreds of different versions on the same articles. The problem with article spinning software is that I have not come across any that can produce high quality content, most spun articles have grammatical errors. The only way to create quality spun articles is to spin by hand which can take as much time as re-writing the article completely.
This is something you should pay particular attention to if you use PLR articles. Always re-write the article and then check it is unique by checking it on copyscape.com
Also check to see if any of your website pages are in the supplemental index.
There are a few ways to check this by I found the most reliable way is to do the following.
Check the total number of pages indexed on your site, type into Google
Check the total number of pages in the Main Index type into Google
Then subtract the number in the main index from the total number of pages indexed to find out how many of your pages are in the supplemental index.
There are other reasons your pages might be in the supplemental index, duplicate content is just one.
As always, Google loves unique quality content so make sure this is what you are providing on your website or blog.