(1) Mass use of the canonical tag. (2) Canonical tag pointing to itself in a same page. How does each engine differ? (1) - Google doesn't punish you if you are using the canonical tag on hundreds or thousands of your site pages, - Bing says the tag is less to good. (2)- Google doesn't care if you link a canonical tag to the same page (i.e. a redirect back to the redirect), - Bing does and says don't do it. Nick quotes Bing's Duane who wrote, "the rel=canonical is that it was never intended to appear across large numbers of pages" and "Its best to leave them [canonical tags] blank rather than point them at themselves." Nick then quotes Google's Cutts and Ohye who say, "it doesn’t hurt to have this on every page of your site," and "it's absolutely okay to have a self-referential rel="canonical". " For Example: Add a rel="canonical" link to the <head> section of each HTML page. <link rel="canonical" href="URL"/>
Almost all of them accept the same format. Basically this is the part which were tried up to fight from the duplicate content pages so they doesn't needed to index or focus upon more pages..