How can I send the mail once?

You can use meta to create a one time action

something like this

$order_id = $order->get_id();

if (get_post_meta($order_id, 'is_mail_sent', true)) return;

update_post_meta($order_id, 'is_mail_sent', true);

Add this right after the opening of the if block

This code will first check if the order meta value of is_mail_sent is true, if true it will return (exit) from the funcion.

If it was not true, first time, then it will continue to the rest of the code and also set the is_mail_sent to true to prevent future mail sending

You could also do this

$order_id = $order->get_id();

if($now>$last_time && !get_post_meta($order_id, 'is_mail_sent', true)) {
    update_post_meta($order_id, 'is_mail_sent', true);

    $order_number = $order->get_order_number(); 
    $to = '[email protected]';
    $subject="Failed order ";
    $body = '#'.$order_number. ' order number failed;
    $headers = array('Content-Type: text/html; charset=UTF-8');
    wp_mail( $to, $subject, $body, $headers );
}

This will perform the same check but will only wrap the mail part of the code

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