Don’t change Custom Post Type slug to unique value

you might consider the ‘wp_unique_post_slug’ filter:

   add_filter( 'wp_unique_post_slug','my_disable_unique_slug',11,6);
    function disable_unique_slug( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ){
        global $post;       
        if($post->post_type=='cpt'){ // EDIT post type
            $slug=$original_slug;
        }
        return $slug;
    }

This is untested, but you get the idea 😉

ps: you might have to change other things to make this work, since the unique-ness is so important.