Magento 错误:标头已发送

I got this error while adding products to the cart in my site:

Cannot send headers; headers already sent in /home/website/public_html/app/code/local/Perpetual/MultiAdd/controllers/Checkout/CartController.php, line 153
Trace:
#0 /home/website/public_html/lib/Zend/Controller/Response/Abstract.php(147):      Zend_Controller_Response_Abstract->canSendHeaders(true)
#1 /home/website/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(585): Zend_Controller_Response_Abstract->setRedirect('http://www.trum...')
#2 /home/website/public_html/app/code/core/Mage/Checkout/controllers/CartController.php(85): Mage_Core_Controller_Varien_Action->_redirect('checkout/cart')
#3 /home/website/public_html/app/code/local/Perpetual/MultiAdd/controllers/Checkout/CartController.php(203): Mage_Checkout_CartController->_goBack()
#4 /home/website/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(376): Perpetual_MultiAdd_Checkout_CartController->addmultipleAction()
#5 /home/website/public_html/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(248): Mage_Core_Controller_Varien_Action->dispatch('addmultiple')
#6 /home/website/public_html/app/Mage.php(459): Mage_Core_Controller_Varien_Front->dispatch()
#7 /home/website/public_html/wholesale/index.php(65): Mage::run('wholesale', 'website')
#8 {main}

禁用无关的插件并没有解决问题.

$url = $this->_getSession()->getRedirectUrl(true)
if ($url) {
    $this->getResponse()->setRedirect($url);
} else {
$this->_redirectReferer(Mage::helper('checkout/cart')->getCartUrl());
}