You specify the main URL since the internet search engine could have problem pinpointing which just one needs to be the first URL.  The next tutorial will provide you with ways to allow Canonical URLs in Prestashop 1.five. Improve the indexing of the Prestashop urls in Google. This module https://addons.prestashop.com/fr/seo-referencement-naturel/31857-seo-automatique-google-rich-snippets-json-ld-open-graph.html