class myClass
{
    protected static int $handlersInProcess = 0;

    public static function addHandlerProcess(): void
    {
        self::$handlersInProcess++;
    }

    public static function deleteHandlerProcess(): void
    {
        self::$handlersInProcess--;
    }

    public static function hasHandlersInProcess(): bool
    {
        return (self::$handlersInProcess > 0);
    }

    public static function iblockElementUpdateHandler(&$fields): void
    {

        if (self::hasHandlersInProcess() === true)
            return;

        self::addHandlerProcess();

        /*  наш код, приводящий к вызову CIBlockElement::Update */
        ...
        CIBlockElement :: Update (..., ...);

        self::deleteHandlerProcess();
    }
}
 

class myClass
{
    protected static bool $handlerInProcess = false;

    public static function iblockElementUpdateHandler(&$fields): void
    {

        if (self::$handlerInProcess === true)
            return;

        self::$handlerInProcess = true;

        /*  наш код, приводящий к вызову CIBlockElement::Update */
        CIBlockElement :: Update (..., ...);

        self::$handlerInProcess = false;
    }
}