I see more and more websites that have very similar pages (if not exact duplicates) which can be reached by many different URLs. For example, a hotel website may have a variety of categories such as "on the beach," "romantic hotels," "hotel villas," "luxury hotels," etc. where many of the hotels belong in more than one category.
Depending on how a user browsed through the site, they may end up at the same hotel page, but with a slightly different URL such as the following:
This is what’s known in the SEO industry as “canonical issues.”
One of the main reasons why having the same content across multiple URLs is such a problem is that it splits the link popularity (and PageRank) of those pages, which provides them with less relevancy weighting in Google. Less weighting means that the page will have less chance of showing up when someone is searching at Google using keyword phrases that are relevant to that page.
What’s the Fix?
The fix has always been to make sure that every hotel page has just one landing page with one, consistent URL. But that’s not always easy within the confines of some content management systems (CMS).
This past year, however, Google's Matt Cutts announced a new HTML "tag" known as the "Canonical Link Element" which was created as an easier fix for this duplicate content issue. While it’s always better to fix your CMS when you can, this new element allows any webmaster to tell the search engines which URL is the preferred one, if and when they find similar content being shown at different URLs.
The code itself goes into the <HEAD> section of the page and looks like this:
<link rel="canonical" href="http://example.com/page.html"/>
Canonical issues have plagued webmasters, developers, and SEOs for ages, so it's great that all of the major search engines have announced that they will support this element.
If your website has canonical issues which you cannot fix by technical means, I highly suggest using this new element, as it will preserve each page’s link popularity by keeping it all within the confines of one URL that the you specifically choose.