The tag will never conceal real duplicate content that you'll have on your web site, for example equivalent item descriptions for various objects. Google will only go through via your hreflang tag that content “a” is often a translation of material “b.” Exam your Nginx configuration for syntax glitches using https://addons.prestashop.com/pl/seo-pozycjonowanie-naturalne/30904-automatyczne-tagi-hreflang-i-tagi-kanoniczne-seo.html