Prevent duplicate WP custom CPT post slugs. This will append the string '-duplicate' to the end of any duplicate slugs. This code cannot prevent duplicate slugs if they already exist prior to implementing this solution. Be sure to check for duplicates first.

gistfile1.txt

function prevent_slug_duplicates( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ) {
    $check_post_types = array(
        'post',
        'page',
        'custom_post_type'
    );
    
    if ( ! in_array( $post_type, $check_post_types ) ) {
        return $slug;
    }

    if ( 'custom_post_type' == $post_type ) {
        // Saving a custom_post_type post, check for duplicates in POST or PAGE post types
        $post_match = get_page_by_path( $slug, 'OBJECT', 'post' );
        $page_match = get_page_by_path( $slug, 'OBJECT', 'page' );

        if ( $post_match || $page_match ) {
            $slug .= '-duplicate';
        }
    } else {
        // Saving a POST or PAGE, check for duplicates in custom_post_type post type
        $custom_post_type_match = get_page_by_path( $slug, 'OBJECT', 'custom_post_type' );

        if ( $custom_post_type_match ) {
            $slug .= '-duplicate';
        }
    }

    return $slug;
}
add_filter( 'wp_unique_post_slug', 'prevent_slug_duplicates', 10, 6 );