Php swiftmailer的新功能

我是swiftmailer的新手，我有这种错误

致命错误：无法在第17行的/home/hosting/public\u html/lib/classes/Swift/Mime/Headers/DateHeader.php中重新声明类Swift\u Mime\u Headers\u DateHeader

我不知道该怎么办，请帮忙

我的代码是：

 require_once 'lib/swift_required.php';

    // Create the Transport
    $transport = Swift_SmtpTransport::newInstance('smtp.example.org', 25)
      ->setUsername('your username')
      ->setPassword('your password')
      ;

    /*
    You could alternatively use a different transport such as Sendmail or Mail:

    // Sendmail
    $transport = Swift_SendmailTransport::newInstance('/usr/sbin/sendmail -bs');

    // Mail
    $transport = Swift_MailTransport::newInstance();
    */

    // Create the Mailer using your created Transport
    $mailer = Swift_Mailer::newInstance($transport);

    // Create a message
    $message = Swift_Message::newInstance('Wonderful Subject')
      ->setFrom(array('john@doe.com' => 'John Doe'))
      ->setTo(array('receiver@domain.org', 'other@domain.org' => 'A name'))
      ->setBody('Here is the message itself')
      ;

    // Send the message
    $result = $mailer->send($message);