Many webmasters may have causally come across mentions in the forums and blogs of how having duplicate content can be bad for a site's SEO performance. They may worry about what if someone copied my site's content and published it elsewhere. Or what if I'm using Wordpress, or other content management system like Joomla or Drupal, where the system auto-generates different URL structures that can lead to the same content.
However, if you research deeper into the matter, duplicate content may not be as big of a concern as some people might be lead to believe. This article attempts to follow the suggestion of Google where they say "You can help your fellow webmasters by not perpetuating the myth of duplicate content penalties!"
The truth is that if you have no malicious intent, then Google does not give you penalty for having duplicate content on the internet.
In the first case of someone republishing your content either manually or automatically scraping, Google is typically able figure out the original source and will list that in its search engine results page and filter out the copied content. How Google knows this is probably a trade secret; but I believe they do know. Since they are constantly monitoring all the pages in the internet, the would have in their records which page had been publish first as well as how often that page changes. As mentioned in their Webmaster Central Blog, they say "we look at various signals to determine which site is the original one, which usually works very well. This also means that you shouldn't be very concerned about seeing negative effects on your site's presence on Google if you notice someone scraping your content."
It is very common to build sites using content management systems like Wordpress, Joomla, Drupal, and others. It is entirely possible that in such systems there are two distinctly different URLs that leads to the same page content. This is considered duplicate content in the eyes of a search engine. However, you are not penalized for this type of duplicate content.
Google Webmaster Central Blog says "Having this type of duplicate content on your site can potentially affect your site's performance, but it doesn't cause penalties. ... This type of non-malicious duplication is fairly common, especially since many CMSs don't handle this well by default. So when people say that having this type of duplicate content can affect your site, it's not because you're likely to be penalized; it's simply due to the way that web sites and search engines work."
If you do not have any malicious intent and if you do not intentionally duplicate content, then generally speaking Google will not penalize you as long as you are providing original and added-value content. Google is smart enough to know whether the duplicate content was copied and republished from your site. They also understands that certain CMS system may generate different URL structures that point to the same content on the same domain name. In both situations, the webmaster of the original content is not at fault. Google understands this and hence will not negatively penalize your SEO ranking.
That is not to say that your SEO ranking can not be further optimized by removing the duplicate URL structures if possible -- Google has some suggestions linked here. There is the slight negative effect of spreading out your page rank when you have multiple URLs going to the same content (read details on SEOBook.com). And there is the added overhead of inefficient GoogleBot crawling. But it is debatable as to how significant these effect are.
I love the way Google summaries it best here: "If you're a webmaster of beginner-to-intermediate savviness, you probably don't need to put too much energy into worrying about duplicate content, since most search engines have ways of handling it".
