Update: Google has finally published what they call the “Duplicate Content Guidelines” for an insight into what you can and cannot to with regards to duplicate content on a site.
I’m sure you have heard it before, duplicate content can put a huge negative effect on your sites SEO. Google and other search engines do everything they can to omit sites that duplicate themselves, or sites that try to duplicate other more popular sites. There are many ways to check your site against duplicate content and the solution to avoid any omitted results is simple.
Update 15/09/2009: Google no longer considers duplicate content as a penalty, rather a chance that you will get omitted from search results because your content is not as relevant as some else.
How do I know if my site is copied or duplicated?
Copyscape is a great way to check if someone is using the content of your site without permission. Simply enter in your web address and see who has the same content as you. If other sites appear then you have either copied another site (smack on wrist) of have been copied. If you are having problems with people stealing your content then you may want to put one of the Copyscape banners on your site and sign up for a protection plan.
This site however does not pick up duplicate content within your own site so you will need to use some other my suggestions to combat this.
Stopping Duplicate Content Within Your Site: Canonical Tag
Google and other search engines came up with a solution to avoid duplicate content on a site. They created a tag to allow users to tell search engines which copy of your page content is the main original copy. The tag looks like this:
<link rel="canonical" href="http://example.com/page.html"/>
Just insert this tag in between the header tags of your code and replace example.com/page.html with with the URL of the main original copy of your site and that’s it. You will need to repeat this on every page that may have a duplication problem. If you would like to see a quick video tutorial on this then visit Matt Cutts tutorial.
WordPress Users: Lucky enough there are a few plugins available for WP users, “Platinum SEO plugin” allows you to select a Canonical Tag box to automatically activate the tag or if you prefer a stand alone plugin you can try the WordPress SEO plugin by Yoast or the SEO no dulicate plugin.
Another big culprit to duplicate content is site like Digg and Facebook that try to frame your site by posting your page within their page using a frame. These sites try to hijack your content for their own SEO gain so be aware of this. You can add this bit of code into your head tags:
if (top !== self) top.location.href = self.location.href;
If you are using WordPress then download and activate Frame Free plugin (this plugin is no longer updated) which will break free of any sites that are trying to hold your content hostage.