SEO Moz is reporting that I have 50+ pages with a duplicate content issue based on this URL: http://www. f r e d aldous.co.uk/art-shop/art-supplies/art-canvas.html?manufacturer=178
But I have included this tag in the source: rel="canonical" href="http://www.f r e daldous.co.uk/art-shop/art-supplies/art-canvas.html"/>
(I have purposefully added white space to the URLs in this message as I'm not sure about the rules for posting links here)
I though this "canonical" tag prevented the duplicate content being indexed?
is the reporting by SEOMoz wrong or being over cautious?