When i visit my plain url of website its working but in end when i add .html its also working but making dublicate errors and affecting seo how to fix it

when i visit my plain url of website its working but in end when i add .html its also working but making dublicate errors and affecting seo how to fix it

my site url https://www.cartoondb.info/

@Aliff34 This is standard Netlify behavior:

A solution is to specify the canonical url:

Another option would be Edge Functions: