Canonical Error and Warnings Format. MSBuild recognizes the specially formatted errors and warnings that many command-line tools write to the console.
Isn't Harry12 not referring to the "content duplication error" that occurs when e.g. http://mysite.com and http://www.mysite.com get indexed (with the same content of course)?
Hope you have noted as, some sites with www version, non www version and some sites along with index.htm extension. These are said to be Canonical Url. In this case the link juice can be split. To avoid this issue, we need to redirect the website either with www version or non www version permanently. For this 301 redirection can be used. 301 redirection can be always search engine friendly
Canonical error means your website is available without 'www' also which should not happen. You must have only one single domain like www.abc.com. It should not be able to open without www. It should redirect to www.abc.com if anybody opens it without www.