You specify the principal URL since the online search engine might have problems figuring out which one need to be the principal URL.  The subsequent tutorial will tell you about the way to allow Canonical URLs in Prestashop 1.5. Increase the indexing of the Prestashop urls in Google. This https://addons.prestashop.com/en/seo-natural-search-engine-optimization/31857-automatic-google-rich-snippets-json-ld-open-graph-seo.html