I'm seeing this mistake time and time again. New clients with old websites have both uppercase URLs and lowercase URLs.
Google to some extent sees the two and separate entities and of course will treat them as duplicate content.
So always make sure to set this from .htaccess: www.example.com/TEST should always 301 to www.example.com/test
Always choose the lowercase as the main URL structure. Also, some website may not have duplicate URLs but it's still possible to load pages like www.Example.com/tEsT - don't know why someone will type like this buy sometimes people do.
Example: https://www.seoclerks.com/Faq leads to 404 - it should redirect 301 to lowercase URL.