Every few months, webmaster forums discuss if search engines penalize duplicate content.
Duplicate content can happen if web pages publish the same articles, if different domains point to the same web space or if webmasters steal the content of other pages. If two shops sell the same item and use similar shop systems, some product pages can also look like duplicated web pages.
Is there really a penalty for duplicate content?
There are many opinions in the discussion forums but there's no proof that search engines really penalize duplicate content.
If there really was a duplicate content filter then many news web sites that publish AP or
Reuters news would be banned from search engines. For example, you can find many web pages with exactly the same article here. All pages can be found on Google.
However, many people insist that a duplicate content filter exists.
Why do people think that there's a duplicate content penalty?
Some people think that there is a duplicate content penalty because a web page that shows a special article might have a Google PageRank 0 and another web page with the same article might have a Google PageRank 5.
Not all web pages with the same content have the same search engine rankings. If a web site is older than another, if it has better inbound links and if that site has more content than it's likely that it will get better rankings than another page that lists the same article.
That doesn't mean that the web site with the worse ranking has been penalized. It just means that the other web site probably has more links and that the page is more trustworthy to search engines.
Some people think that there must be a duplicate content filter because additional domain names that point to the same web space as the main domain name are usually not listed on search engines.
This is not due to a duplicate content filter. It's an issue with canonical URLs. Google has addressed that problem with their latest ranking algorithm update.
Whether there is a duplicate content penalty or not has yet to be proved. If you want to outperform your competition on search engines, make sure that your web site has unique content that cannot be found on other sites.
If your web site has unique content, you don't have to worry about potential duplicate content penalties. Optimize that content for search engines and make sure that your web site has good inbound links.
It's hard to beat a web site with great optimized content and many good inbound links.