Solve Duplicate Content Issue with Canonical Tag

Duplicate content or vastly similar content that is accessible through multiple URLs has been haunting webmasters for a long time. Discussions on how it affects Search Engine Optimization (SEO) and possible countermeasures to prevent search engines from spelling their wrath over sites with duplicate content continued unabated. There was, however, no definitive solution or no real mechanism being employed by search engines until recent development where canonical tag was first introduced.

Google, Yahoo and Microsoft have finally united in order to reduce duplicate content on the net. With the "collaboration" of the 3 search engines on this particular matter, webmasters can deal the issue with full confidence by making full use of the canonical link tag. This special tag allows you to specifically specify your preferred version of a URL.

This issue may appear a bit confusing to many people without proper example to illustrate it but the following is the example how duplicate content (especially on dynamic sites like forum) may occur because a site might allow various sort orders for a page

1. http://forums.digitalpoint.com/forumdisplay.php?f=5
2. http://forums.digitalpoint.com/forumdisplay.php?f=5&daysprune=-1&order=asc&sort=title
3. http://forums.digitalpoint.com/forumdisplay.php?f=5&daysprune=-1&order=desc&sort=lastpost

From the above example we can clearly see that a particular page can be accessed via multiple URLs. This has several adverse effects on an affected site like less of the site may get crawled and each page may not get full link credit.

Illustrations by Matt Cutts

In order to avoid it, you can simply add the tag to specify your preferred version like the following example

<link rel="canonical" href="http://forums.digitalpoint.com/forumdisplay.php?f=5" />

inside the section of the duplicate content URLs and Google will understand that the duplicates all are pointing to the canonical URL (the specified URL).

This tag will operate in a similar way to a 301 redirect for all URLs that display the page with this tag.

• Links to all URLs will be consolidated to one specified as canonical.
• Search engines will consider this URL a "strong hint" as to the one to crawl and index.

For WordPress users, chill out! A plugin for canonical tag is now available for download. The following is few more detail references on "Canonical Tag" that may interest you.