diff --git a/vendor/magento/module-multicoupon/Model/CouponManagement.php b/vendor/magento/module-multicoupon/Model/CouponManagement.php
index 7f37d005afaf..a290965f52a3 100644
--- a/vendor/magento/module-multicoupon/Model/CouponManagement.php
+++ b/vendor/magento/module-multicoupon/Model/CouponManagement.php
@@ -58,10 +58,10 @@ public function get(int $cartId): array
     {
         $quote = $this->quoteRepository->get($cartId);
         $coupons = $quote->getExtensionAttributes()->getCouponCodes() ?? [];
-        if ($quote->getCouponCode() && !in_array($quote->getCouponCode(), $coupons)) {
+        if ($quote->getCouponCode() && !$this->couponsInList([$quote->getCouponCode()], $coupons)) {
             array_unshift($coupons, $quote->getCouponCode());
         }
-        return $coupons;
+        return array_unique($coupons);
     }

     /**
@@ -74,10 +74,11 @@ public function append(int $cartId, array $couponCodes): void
         }
         $quote = $this->quoteRepository->get($cartId);
         $appliedCouponCodes = $quote->getExtensionAttributes()->getCouponCodes() ?? [];
-        if ($quote->getCouponCode() && !in_array($quote->getCouponCode(), $appliedCouponCodes)) {
-            array_unshift($appliedCouponCodes, $quote->getCouponCode());
+        $appliedCouponCodes = array_map('strtolower', $appliedCouponCodes);
+        if ($quote->getCouponCode() && !in_array(strtolower($quote->getCouponCode()), $appliedCouponCodes)) {
+            array_unshift($appliedCouponCodes, strtolower($quote->getCouponCode()));
         }
-        if (empty(array_diff($couponCodes, $appliedCouponCodes))) {
+        if (!$this->couponsInList($couponCodes, $appliedCouponCodes)) {
             return;
         }
         $allCouponCodes = array_unique(array_merge($appliedCouponCodes, $couponCodes));
@@ -150,10 +151,13 @@ private function validateCouponCodes(CartInterface $quote, array $couponCodes):
             $customerId = (int)$quote->getCustomer()->getId();
         }

-        $invalidCouponCodes = array_diff(
-            $couponCodes,
-            array_values($this->validateCouponCode->execute($couponCodes, $customerId))
-        );
+        $invalidCouponCodes = [];
+        $validCoupons = $this->validateCouponCode->execute($couponCodes, $customerId);
+        foreach ($couponCodes as $code) {
+            if (!in_array(strtolower($code), array_map('strtolower', $validCoupons))) {
+                $invalidCouponCodes[] = $code;
+            }
+        }

         if (!empty($invalidCouponCodes)) {
             throw new InputException(
@@ -197,7 +201,13 @@ private function getNotAppliedCoupons(CartInterface $quote, array $couponCodes):
             $quote->setCouponCode('');
         }

-        return array_diff($couponCodes, $appliedCoupons);
+        $notAppliedCoupons = [];
+        foreach ($couponCodes as $couponCode) {
+            if (!in_array(strtolower($couponCode), array_map('strtolower', $appliedCoupons))) {
+                $notAppliedCoupons[] = $couponCode;
+            }
+        }
+        return $notAppliedCoupons;
     }

     /**
@@ -229,4 +239,21 @@ public function remove(int $cartId, array $couponCodes = []): void

         $this->quoteRepository->save($quote->collectTotals());
     }
+
+    /**
+     * Check if the coupon codes are in the list
+     *
+     * @param array $couponCodes
+     * @param array $couponList
+     * @return bool
+     */
+    private function couponsInList(array $couponCodes, array $couponList): bool
+    {
+        return !empty(
+            array_diff(
+                array_map('strtolower', $couponCodes),
+                array_map('strtolower', $couponList)
+            )
+        );
+    }
 }
diff --git a/vendor/magento/module-multicoupon-ui/Plugin/AdminCreateOrderApplyCoupons.php b/vendor/magento/module-multicoupon-ui/Plugin/AdminCreateOrderApplyCoupons.php
index dfe3eda9d4e6..dfe583bcd0a9 100644
--- a/vendor/magento/module-multicoupon-ui/Plugin/AdminCreateOrderApplyCoupons.php
+++ b/vendor/magento/module-multicoupon-ui/Plugin/AdminCreateOrderApplyCoupons.php
@@ -163,8 +163,8 @@ private function isValidCouponCode(Quote $quote, string $appendCouponCode): bool
         }

         $invalidCouponCodes = array_diff(
-            [$appendCouponCode],
-            array_values($this->validateCouponCode->execute([$appendCouponCode], $customerId))
+            [strtolower($appendCouponCode)],
+            array_map('strtolower', array_values($this->validateCouponCode->execute([$appendCouponCode], $customerId)))
         );
         if (!empty($invalidCouponCodes)) {
             return false;
diff --git a/vendor/magento/module-multicoupon-ui/Plugin/AdminCreateOrderValidateCoupon.php b/vendor/magento/module-multicoupon-ui/Plugin/AdminCreateOrderValidateCoupon.php
index 9aaceb9179db..9eb13fd4929c 100644
--- a/vendor/magento/module-multicoupon-ui/Plugin/AdminCreateOrderValidateCoupon.php
+++ b/vendor/magento/module-multicoupon-ui/Plugin/AdminCreateOrderValidateCoupon.php
@@ -73,7 +73,7 @@ public function aroundExecute(ValidateCoupon $subject, \Closure $proceed, CartIn
             return;
         }

-        if (in_array($code, $quote->getExtensionAttributes()->getCouponCodes())) {
+        if (in_array(strtolower($code), array_map('strtolower', $quote->getExtensionAttributes()->getCouponCodes()))) {
             $this->messageManager->addSuccessMessage(__('The coupon code has been accepted.'));
             return;
         }