This blog post discusses how to create a filter in Google Analytics to clean up URL's with trailing slashes http://www.lunametrics.com/blog/2010/12/17/cleaning-urls-google-analytics/
You may want to view your pages loading and redirecting with a live http header addon or extension for your browser. What type of redirect is happening?
In WordPress with permalinks set viewing my real time analytics connections and going to a URL without the trailing slash forcing WordPress to add the slash and reload the page I do not see a hit to a page without the trailing slash. I also don't have any duplicate content views to pages with and without the trailing slash.
Your shopping cart or app may not be redirecting properly, check the support forums of the software you run and see if anyone has been able to force a trailing slash for relative URL's when they're generated in the template.
Pageviews may also be inaccurate because of the extra views to these pages. Visitors should be accurate since that's counted a different way. How is the conversion rate on the site?