Canonical URL
Google, Yahoo and MSN are now supporting a Canonical URL tag to help manage duplicate content issues. Duplicate content can occur when a product is listed under multiple categories, each with its own URL. Another instance could be when a search engine crawls a printer friendly version of a page.
There are several issues that exist with duplicate content. When multiple copies of a page exist search engines want to choose one version to show and filter out the rest. This could end up with the search engine showing your printer friendly page and almost completely defeating the purpose of your organic results. Your PageRank may also suffer in this case due to the fact that it’s being spread across pages you don’t want ranked well.
Before Canonical URL tags the best way to let search engines know which pages to crawl was to add rel=”nofollow” to the links you didn’t wanted crawled. Now you can use rel=”canonical” to let search engines know which page is the original version. This may not work correctly every time because search engines aren’t perfect, but it will surely help you achieve better results.