You specify the main URL since the search engine can have trouble identifying which a person needs to be the first URL.  The following tutorial will teach you tips on how to allow Canonical URLs in Prestashop 1.5. It may even let you increase the default "x-default" tag for https://addons.prestashop.com/nl/seo-zoekmachineoptimalisatie-vermelding/31857-automatische-google-rich-snippets-json-ld-opengraph-seo.html