An SEO expert reported me that every NopCommerce pages are tagged as duplicated by Google because :
http://domain.com/product and http://domain.com/product/ are treated as a separate page and don't redirect.
The following post from 2014 does a good summary of the problem and give a solution with a URL rewrite rules. https://www.nopcommerce.com/boards/t/28746/url-trailing-slash-for-seo-purposes.aspx
I am using NopCommerce 4.10. Is there a better solution than the URL rewrite to avoid this ?