How do I find the duplicate page when it's not in All Pages?

In a crawl report generated by SEOMoz, a duplicate page content warning has been displayed. The two urls are

1. http://www.superheroes.com.au/shop
and
2. http://www.superheroes.com.au/shop/category/catalog/

I cannot find the second page anywhere in the list of pages in the Admin (so I can add canonicalisation code) When I view the 2nd page and click Edit Page, it redirects to the 1st page.

Any ideas where SEOMoz would be finding this 2nd page or how it might be being generated?

(btw I didn’t build this site)

Thanks

Simon