How to check for a duplicate email address in PHP, considering Gmail (user.name+label@gmail.com)

Question

How can I check for duplicate email addresses in PHP, with the possibility of Gmail\'s automated labeler and punctuation in mind?

For example, I wan

Answer 1

I have extended Zend Validator like this.

<?php
class My_Validate_EmailAddress extends Zend_Validate_EmailAddress
{
    public function isValid($value)
    {
        $valid = parent::isValid($value);
        if ($valid
                && in_array($this->_hostname, array('gmail.com', 'googlemail.com'))
                && substr_count($this->_localPart, '.') > 1) {
            $this->_error(parent::INVALID_HOSTNAME);
            $valid = false;
        }
        return valid;
    }
}

Email with more than one "dot" symbol in gmail address are considered invalid. For some cases this is not logical solution, but that works for me.