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I'm building a new website and am setting up internal link structure with subcategories and hoping to do so with best Seo practices in mind.  When linking to a subcategory's main page, would I make the internal link  www.xxx.com/fishing/ or www.xxx.com/fishing/index.html or does it matter?  I'm just trying to avoid duplicate content I guess, if Google saw each page as a separate page.  Any other cautions when using subdirectories in my navigation?