I just finished watching a video from Matt Cutts about the new canonical link element, and in the video he ran through a checklist of how to fix/avoid duplicate content issues.  All of these come from the video, so I really have to give all the credit here to Matt, but I felt these were worth mentioning.

Before we talk about the ways to fix duplicate content issues, let’s talk about why duplicate content is bad for search engines.  At a very high level, search engines look at duplicate content as a form of web plagiarism.  If two sites have pages with exactly the same content, the search engine will try to figure out which page has the most authority and trust with regard to the topic.  This is because they only want to serve one version of the content in the search results, since users really only need to see one version of the exact same content.

Duplicate content, however, is often a serious site architecture issue within a single site.  Duplicate content can exist within a site for a variety of reasons, sometimes as the result of using session IDs, tracking parameters, breadcrumb trails, etc.  In these types of cases, we are really just cannibalizing our own success with these duplicate content issues.

In his video, Matt Cutts described a variety of ways to fix duplicate content issues.

1. Customize CMS systems or sites in the beginning – By customizing a CMS system to only produce the URLs that you want, you can prevent a lot of duplicate content issues before they even start.
2. Link only to one version of a page – Google finds new pages and new sites by scanning existing web pages, identifying links to other pages, and then spidering those pages.  If you only link to one version of a page, a search engine can (theoretically) only identify that page from that singular URL.  Of course, in practice this is incredibly difficult, since you cannot control how other sites will choose to link to your site.
3. Use 301 redirects – Most of the time, this is probably going to be the most likely method to realistically fix duplicate content issues.  Using a 301 redirect tells Google that a given web page has been moved to a different URL.  The user, search engines included, will be redirected to your specified URL, thereby not seeing the duplicate content.
4. Webmaster Tools – Webmasters can use Google’s Webmaster Tools to specify which version of the top level domain you wish to show to users.  While this allows you to specify either a www or non-www preference, thereby solving duplicate content issues at the domain level, it does not really solve duplicate content issues at deeper levels (for example, if the duplicate content is the result of tracking parameters).
5. Use site map to list preferred URLs – Webmasters can submit an XML site map to Google to show the search engine which URLs are the preferred URLs on the site.
6. Use the canonical link element – Much like a 301 redirect, the canonical link element allows webmasters to specify to the search engines which page is the preferred page and which page contains duplicate content at a page-by-page level.  To use the canonical link element, place the element in the head section of the page code for the non-preferred page.  Then, use the element to specify the URL for the preferred page displaying the content.

So what’s the difference between using a 301 redirect and using a canonical link element?  The 301 redirect will actually redirect the user to the preferred page on your site, bypassing the non-preferred page and resulting in the preferred page’s URL resolving in the users browser.  The canonical link element will not redirect the user, but will simply tell the search engines that this specific page is the non-preferred page with very similar or identical content that can be found on another page on the site.

Let’s say you are a widget maker, and you just came out with your shiny new 2009 widgets.  Since the 2009 widget is pretty much the same as the 2008 widget, and you completely sold out of your stock of 2008 widgets, you have decided to simply replicate your marketing copy from the 2008 widgets page to a 2009 widgets page.  Your site is not dynamically creating the page, and you aren’t using session IDs or tracking parameters.  In this case, you will probably be best using a 301 redirect from the old 2008 widgets URL to the new 2009 widgets URL.

However, if you were dynamically using tracking parameters to create the landing page, or needed to pass through some session IDs in order to track the user, you would probably want to use the canonical link element.  This is because you still want to maintain the tracking through the site, while still letting the search engines know that the session IDs are creating duplicate content.

Hopefully this helps to offer some insight on how to fix duplicate content issues, as well as to clear up any confusion on when to use the new canonical link element versus a 301 redirect.  Remember, if you can take some steps to reduce the possibility of duplicate content issues early on, you’ll be helping yourself out tremendously.  But if you can’t, there are a handful of different ways to make the fix.