Magento log_visitor为单个会话填充多个记录

Question

I have added new column in Magento's log_visitor table to track visitor's utm parameters. But after I have added the column multiple records get inserted for new visitor. Below is my code written in a file \app\code\core\Mage\Log\Model\Resource\Visitor.php

protected function _saveVisitorInfo($visitor)
{

    $stringHelper = Mage::helper('core/string');

    $referer    = $stringHelper->cleanString($visitor->getHttpReferer());
    $referer    = $stringHelper->substr($referer, 0, 255);
    $userAgent  = $stringHelper->cleanString($visitor->getHttpUserAgent());
    $userAgent  = $stringHelper->substr($userAgent, 0, 255);
    $charset    = $stringHelper->cleanString($visitor->getHttpAcceptCharset());
    $charset    = $stringHelper->substr($charset, 0, 255);
    $language   = $stringHelper->cleanString($visitor->getHttpAcceptLanguage());
    $language   = $stringHelper->substr($language, 0, 255);



    $adapter = $this->_getWriteAdapter();
    $data = new Varien_Object(array(
        'visitor_id'            => $visitor->getId(),
        'http_referer'          => $referer,
        'http_user_agent'       => $userAgent,
        'http_accept_charset'   => $charset,
        'http_accept_language'  => $language,
        'server_addr'           => $visitor->getServerAddr(),
        'remote_addr'           => $visitor->getRemoteAddr(),
        'utm_name'              => "utm_name",
    ));



    $bind = $this->_prepareDataForTable($data, $this->getTable('log/visitor_info'));

    $adapter->insert($this->getTable('log/visitor_info'), $bind);


    return $this;
}

I have just added 'utm_name' and because of this multiple records are getting inserted.