Have you moved servers recently? In years past, Windows servers always treated them differently than on unix servers.
Barring that, you can edit your .htaccess file to convert all URLs to lowercase. Here’s a guide editing depending if you have access to httpd.conf file or not;
https://www.rewriteguide.com/apache-enforce-lower-case-urls/
As suggested by @Rup, a canonical URL should take care of it all too.
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