koan - 2:32 am on Jul 26, 2011 (gmt 0)
It may not be your fault at all, just other sites linking to you, adding parameters for their own tracking needs. Some of my sites in Webmaster Tool have duplicate title problems because Google found links that always add &referer=sphere_search at the end. It's annoying and I haven't figured out how to fix it without doing some PHP 301 redirect for every possible url parameters that shouldn't be there. The canonical approach is interesting.