duplicate content

Just like the lamb lying down with lion, but the big three search engines came together for a rare joint effort to announce the launch of a new feature that will help ease the world’s duplicate content problems. The new feature, called the tag tells the search engines what version of the URL is the correct one to want index.
The link tag goes in the section of the page of looks like this:

The search engines have been doing a pretty good job figuring out the right URL when people use redirects properly, but it’s not always easy to get it right–especially when we build these nutty sites with 50 different URLs pulling up the same content. This change should help a lot of webmasters sleep easier knowing that their proper, canonical URL will be indexed and given all the link juice it deserves. I look at it like a page by page sitemap to tell the search engines what URL to index.
Official blog posts about the new link / canonical tag:
Yahoo
Google
Microsoft
Coverage on other blogs:
Search Engine Land
Joost already created a WordPress plugin for this feature
3 Reasons to Use rel=canonical, 4 Reasons not to use it