Page get parameter doesn’t work with is_single() function

If exist the page parameter in the blog post, I have to redirect to the 404 page instead of current post URL.
I tried to check the current page type (is singular) and try to get page parameter in the function.php, but I couldn’t. It’s doesn’t work together. I need to implement it only for blog post.
If possible .htaccess redirect also fine

Double domain name in category URL-s

My site runs on the latest version of WordPress. However, I have a problem with categories. Permalinks are set correctly and my category path looks like this: example.com/category-name. Links work fine, but the problem is, I can also access every category with a link like this: example.com/example.com/?cat=6, which contains no posts.