i see no code on your page that would replace the coupon in the usercoupons table.
i only see code on the page to write the user coupon session:
<p>As a <strong>special thank you for registering</strong>,<br/>here is your <strong>20% Off Coupon Code</strong>:<span style="color:#609
"> <?php echo $_SESSION['CouponCode']; ?></span><br/>The code <strong>may be used only once</strong> and will<br/>
"><strong>deduct 20% from your order total</strong></span>.<br/>
you could add the same if statement around that code to not display the coupon once they have already verified.