I have a multilanguage site with urls like this
https://example.org/en/ads/2 https://example.org/es/ads/2 https://example.org/it/ads/2
which serve the site in different languages. My site also accepts URLs like this
In that case the content is served in the default language (Spanish).
The site does not generate any URLs without locale. It accepts this kind of URLs, but then any following links for internal navigation are generated with the default locale (es).
I've been told that this is bad for SEO because different URLs (https://example.org/ads/2, https://example.org/es/ads/2) serve the same content.
Is this true? If yes, why?