WordPress doesn't remember my custom rewrite rule

Question

I have the following code:

function b09_custom_rewrite_rule() {
    add_rewrite_rule(
      'utils/([^/]+)/?',
      'index.php?utils=$matches[1]',
      'top'
    );
    flush_rewrite_rules();
}
add_action('init', 'b09_custom_rewrite_rule', 10, 0);

function b09_query_vars( $query_vars ) {
    array_push( $query_vars, 'utils' );
    return $query_vars;
}
add_filter( 'query_vars', 'b09_query_vars' );

function b09_template_redirect(){
    global $wp_query;

    $utils_page = get_query_var("utils");

    var_dump( $utils_page );

    if( $utils_page ):
        // custom function
        exit();
    endif;
}
add_filter( 'template_redirect', 'b09_template_redirect' );

Everything works as expected, but as soon as I remove the line flush_rewrite_rules(), WordPress is disabling the rule on the next reload.

Any Ideas?

Answer 1

Thanks @cybmeta for your hint. I searched my theme and found an other flush_rewrite_rules(true); on the init hook. I removed this and now the rule is being remembered.

Seemingly, calling the function twice is breaking something. Doesn't really make sense to me, but it works now, so whatever :)