Issue #3414323: Added the checkbox for allow_multiple_coupons field and added...

Issue #3414323: Added the checkbox for allow_multiple_coupons field and added the method in the promotion entity class and interface

Closes #3414323

modules/promotion/src/Entity/Coupon.php

@@ -226,6 +226,18 @@ class Coupon extends CommerceContentEntityBase implements CouponInterface {
       return FALSE;
     }
 
+    $promotion = $this->getPromotion();
+    $coupons = $order->get('coupons')->referencedEntities();
+    foreach ($coupons as $coupon) {
+      if (
+        $this->id() !== $coupon->id() &&
+        $promotion->id() === $coupon->getPromotionId() &&
+        !$promotion->isMultipleCouponsAllowed()
+      ) {
+        return FALSE;
+      }
+    }
+
     $usage_limit = $this->getUsageLimit();
     $usage_limit_customer = $this->getCustomerUsageLimit();
     // If there are no usage limits, the coupon is available.