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// http://ka.lpe.sh/2012/07/26/magento-check-if-customer-already-exist-or-not/

protected function _customerExists($email, $websiteId = null)
{
    $customer = Mage::getModel('customer/customer');
    if ($websiteId) {
        $customer->setWebsiteId($websiteId);
    }
    $customer->loadByEmail($email);
    if ($customer->getId()) {
        return $customer;
    }
    return false;
}