Canonical tags, also known as canonical link elements, are one way to help straighten things out. They have been covered in a previous post, but in this case they can be a lifesaver when dealing with all the strange things that a search engine can do when it tries to figure out which pages to index and where.

For starters, it pays to have a self-referencing tag on each page of your site. If you prefer the www version of your site, then you should reference that in the tag itself. Even though it may seem redundant and unnecessary for pages to reference themselves with a canonical link element, the value of this approach pays off in cases where URLs are appended with tracking data, or other sites point to yours. In one case, we saw a company that had a collection of domain names based on common misspellings of their brand name. Each domain pointed to the main site (instead of 301 redirecting) and this was creating a hodgepodge of indexed pages. Since a canonical link element references the "true" URL, a search engine does not need to be confused, whether the domain in question is yours or not.

Duplicate content from printer friendly pages should be handled a bit differently, since you are basically referencing the actual page on the link element for the printer friendly one. Most webmasters handle printer friendly pages by segregating them in a directory of their own which is then disallowed by the robots.txt file. However, people sometimes link to printer-friendly pages, so it pays to tell the search engine where the spiderable content exists. There is also a belief in the SEO community that a canonical link element works like a "mini 301 redirect" so if this is true it makes sense to include this tag on any page that is duplicating content from elsewhere on your own site. Because these tags work across websites, you may also include the tag on pages that get syndicated or copied elsewhere.

The fight against duplicate content is an important one for any webmaster, since there are so many ways to inadvertently copy your own information elsewhere, even without having to deal with plagiarism and wholesale copying by competitors and scraper sites. Preventing and controlling content for a search engine also gives you site a more unified profile in the engines, and can be a distinct advantage when your content is stacked up against other sites which may have other factors in their favor, but aren't organized in a way that a search engine can understand.