WordPress.org

Inserts canonical link code into header. After activation, from your WordPress Settings, change Permalinks to Custom Structure: /%postname%/ and you're done, no further steps are needed.

If you have any problems please visit: http://wordpress.org/extend/plugins/canonical-link/installation/ for full detailed instructions.

The importance of the canonical link is to give Google a clean representation of what you want your URLs to look like and how you want Google to index them, this can also help to clear up duplicate page issues.

http://bryanhadaway.com/super-simple-dynamic-canonical-link-code/