commit 83bb6ba45520f896e208dd06d7eda34c57ad1898
Author: Joel Pittet <joel@pittet.ca>
Date:   Wed Oct 21 12:20:59 2015 -0700

    code

diff --git a/commerce_coupon.module b/commerce_coupon.module
index 52b84ba..1da358f 100644
--- a/commerce_coupon.module
+++ b/commerce_coupon.module
@@ -773,7 +773,7 @@ function commerce_coupon_redeem_coupon_code($code, $order, &$error) {
 
   if ($coupon) {
     // The same coupon cannot be added twice.
-    if (commerce_coupon_order_has_coupon_code($coupon->coupon_id, $order)) {
+    if (commerce_coupon_order_has_coupon_code($coupon->code, $order)) {
       $error = t('The coupon you have entered has already been applied to your order');
       return;
     }