<controller_action_postdispatch_checkout_onepage_saveBilling><!-- when a billing address is saved check for double discounts -->
            <observers>
                <yourmodule_observer_check_double_discount>
                    <type>singleton</type>
                    <class>namespace_yourmodule/observer</class>
                    <method>checkForDoubleDiscount</method>
                </yourmodule_observer_check_double_discount>
            </observers>
</controller_action_postdispatch_checkout_onepage_saveBilling>

public function checkForDoubleDiscount($observer) {

    $sess = Mage::getSingleton('checkout/session');
    $quote = $sess->getQuote();

    //check if quote has discount
    $coupon_code = $quote->getCouponCode();
    if($coupon_code){
        //check if email address is set
        if($email = $quote->getCustomerEmail()) {

                //get salesrule
                $coupon = Mage::getModel('salesrule/coupon')->load($coupon_code, 'code');
                $rule = Mage::getModel('salesrule/rule')->load($coupon->getRuleId());

                //check if usage limit exists
                if($maxuse = $rule->getUsesPerCustomer()) {

                    //get all orders of the customer
                    $order = Mage::getModel('sales/order');
                    $collection = $order->getCollection()
                   ->addAttributeToFilter('customer_email',array('like'=>$email))
                   ->addAttributeToFilter('coupon_code',array('like'=>$coupon_code));
                    $count = 0;
                    foreach($collection as $o){
                        //check if coupon code has been applied
                        if(strtolower($o->getCouponCode()) == strtolower($coupon_code)) $count ++;
                        //if max usage is reached, remove rule from quote
                        if($maxuse <= $collection->count()) {
                            $quote->setCouponCode('')->collectTotals()->save();
                            $message = Mage::helper('namespace_yourmodule');->__('Coupon Code %s has been cancelled, the usage limit has been reached', $coupon_code);
                            $result = array();
                            $result['error'] = '-1';
                            $result['message'] = $message;
                            print Mage::helper('core')->jsonEncode($result); exit;
                            break;
                        }
                    }
                }
            }
        }
    }
}