You specify the principal URL because the internet search engine may have problem identifying which one particular needs to be the first URL.  The following tutorial will demonstrate tips on how to allow Canonical URLs in Prestashop 1.5. Improve the indexing of your Prestashop urls in Google. This module https://addons.prestashop.com/pt/seo-referenciamento-natural/31857-rich-snippets-automaticos-json-ld-opengraph-seo.html