prevent custom field value duplication

Hi everyone!

I have problems with the query which check current meta-key value, and prevent before duplicate field.

Actually i have this

function wpufe_isbn_validation( $errors ) {
    if( $_POST['cf_isbn'] == 'xxxxxxx' ) {
        $errors[] = 'this ISBN is already in database';
    }

    return $errors;
}
add_filter( 'wpuf_add_post_validation', 'wpufe_isbn_validation' );

All I need is query to database to get all ISBN values that already exist in database. After this my front end form called cf_isbn is compared with exist value from database, and if isbn is already in database it shows the error.

Thanks for help!