Coupon is always re-added during checkout in magento 1.9

I encountered the following bug

Add a product to cart
Apply a coupon code
Remove coupon code
Coupon code is not shown anymore / remove successful
Go in checkout
Continue billing and shipping step
Go back in cart
Coupon code of step #2 is active again
magento magento-1.9


I solved it by event/observer

<events>
        <controller_action_predispatch_checkout_cart_couponPost>
            <observers>
                <remove_session_coupon_code>
                    <type>singleton</type>
                    <class>yourmodule/observer</class>
                    <method>removeCoupon</method>
                </remove_session_coupon_code>
            </observers>
        </controller_action_predispatch_checkout_cart_couponPost>
    </events>


public function removeCoupon(Varien_Event_Observer $observer)
{
    $controller = $observer->getControllerAction();
    if ($controller->getRequest()->getParam('remove') == 1) {
        Mage::getSingleton("checkout/session")->unsetData('cart_coupon_code');
    }
    return $this;
}