diff --git a/src/main/java/com/leungcheng/spring_simple_backend/domain/order/Order.java b/src/main/java/com/leungcheng/spring_simple_backend/domain/order/Order.java
index b9daf39..675ca5f 100644
--- a/src/main/java/com/leungcheng/spring_simple_backend/domain/order/Order.java
+++ b/src/main/java/com/leungcheng/spring_simple_backend/domain/order/Order.java
@@ -1,5 +1,6 @@
 package com.leungcheng.spring_simple_backend.domain.order;
 
+import jakarta.persistence.Column;
 import jakarta.persistence.Entity;
 import jakarta.persistence.Id;
 import jakarta.persistence.Table;
@@ -11,11 +12,15 @@ public class Order {
   private String buyerUserId;
   private PurchaseItems purchaseItems;
 
+  @Column(unique = true)
+  private String requestId;
+
   private Order() {}
 
-  Order(String buyerUserId, PurchaseItems purchaseItems) {
+  Order(String buyerUserId, PurchaseItems purchaseItems, String requestId) {
     this.buyerUserId = buyerUserId;
     this.purchaseItems = purchaseItems;
+    this.requestId = requestId;
   }
 
   public String getId() {
@@ -29,4 +34,8 @@ public String getBuyerUserId() {
   public PurchaseItems getPurchaseItems() {
     return purchaseItems;
   }
+
+  public String getRequestId() {
+    return requestId;
+  }
 }
diff --git a/src/main/java/com/leungcheng/spring_simple_backend/domain/order/OrderRepository.java b/src/main/java/com/leungcheng/spring_simple_backend/domain/order/OrderRepository.java
index 14765ce..d35a779 100644
--- a/src/main/java/com/leungcheng/spring_simple_backend/domain/order/OrderRepository.java
+++ b/src/main/java/com/leungcheng/spring_simple_backend/domain/order/OrderRepository.java
@@ -1,5 +1,8 @@
 package com.leungcheng.spring_simple_backend.domain.order;
 
+import java.util.Optional;
 import org.springframework.data.repository.CrudRepository;
 
-public interface OrderRepository extends CrudRepository<Order, String> {}
+public interface OrderRepository extends CrudRepository<Order, String> {
+  Optional<Order> findByRequestId(String requestId);
+}
diff --git a/src/main/java/com/leungcheng/spring_simple_backend/domain/order/OrderService.java b/src/main/java/com/leungcheng/spring_simple_backend/domain/order/OrderService.java
index baafaed..e768853 100644
--- a/src/main/java/com/leungcheng/spring_simple_backend/domain/order/OrderService.java
+++ b/src/main/java/com/leungcheng/spring_simple_backend/domain/order/OrderService.java
@@ -28,7 +28,12 @@ public CreateOrderException(String message) {
 
   @Retryable(noRetryFor = CreateOrderException.class)
   @Transactional(isolation = Isolation.SERIALIZABLE)
-  public Order createOrder(String buyerUserId, PurchaseItems purchaseItems) {
+  public Order createOrder(String buyerUserId, PurchaseItems purchaseItems, String requestId) {
+    Optional<Order> order = orderRepository.findByRequestId(requestId);
+    if (order.isPresent()) {
+      return order.get();
+    }
+
     User buyer =
         getUser(buyerUserId).orElseThrow(() -> new CreateOrderException("Buyer does not exist"));
 
@@ -39,7 +44,7 @@ public Order createOrder(String buyerUserId, PurchaseItems purchaseItems) {
     }
     saveNewBalance(buyer, buyer.getBalance().subtract(totalCost));
 
-    return addNewOrder(buyerUserId, purchaseItems);
+    return addNewOrder(buyerUserId, purchaseItems, requestId);
   }
 
   private Optional<User> getUser(String userId) {
@@ -51,8 +56,8 @@ private void saveNewBalance(User buyer, BigDecimal newBalance) {
     userRepository.save(updatedBuyer);
   }
 
-  private Order addNewOrder(String buyerUserId, PurchaseItems purchaseItems) {
-    Order order = new Order(buyerUserId, purchaseItems);
+  private Order addNewOrder(String buyerUserId, PurchaseItems purchaseItems, String requestId) {
+    Order order = new Order(buyerUserId, purchaseItems, requestId);
     return orderRepository.save(order);
   }
 
diff --git a/src/test/java/com/leungcheng/spring_simple_backend/domain/order/OrderServiceTest.java b/src/test/java/com/leungcheng/spring_simple_backend/domain/order/OrderServiceTest.java
index 6b21dc0..13ede40 100644
--- a/src/test/java/com/leungcheng/spring_simple_backend/domain/order/OrderServiceTest.java
+++ b/src/test/java/com/leungcheng/spring_simple_backend/domain/order/OrderServiceTest.java
@@ -57,8 +57,7 @@ void shouldRejectCreateOrderWithNonExistingBuyer() {
 
     CreateOrderException exception =
         assertThrows(
-            CreateOrderException.class,
-            () -> orderService.createOrder("non_existing_buyer_id", purchaseItems));
+            CreateOrderException.class, () -> createOrder("non_existing_buyer_id", purchaseItems));
     assertEquals("Buyer does not exist", exception.getMessage());
   }
 
@@ -70,9 +69,7 @@ void shouldRejectCreateOrderWithEmptyPurchaseItems() {
     PurchaseItems purchaseItems = new PurchaseItems();
 
     CreateOrderException exception =
-        assertThrows(
-            CreateOrderException.class,
-            () -> orderService.createOrder(buyer.getId(), purchaseItems));
+        assertThrows(CreateOrderException.class, () -> createOrder(buyer.getId(), purchaseItems));
     assertEquals("Purchase items cannot be empty", exception.getMessage());
   }
 
@@ -85,9 +82,7 @@ void shouldRejectCreateOrderWithNonExistingProduct() {
     purchaseItems.setPurchaseItem("non_existing_product_id", 1);
 
     CreateOrderException exception =
-        assertThrows(
-            CreateOrderException.class,
-            () -> orderService.createOrder(buyer.getId(), purchaseItems));
+        assertThrows(CreateOrderException.class, () -> createOrder(buyer.getId(), purchaseItems));
     assertEquals("Product: non_existing_product_id does not exist", exception.getMessage());
   }
 
@@ -103,9 +98,7 @@ void shouldRejectCreateOrderWithInsufficientBalance() {
     purchaseItems.setPurchaseItem(product.getId(), 2);
 
     CreateOrderException exception =
-        assertThrows(
-            CreateOrderException.class,
-            () -> orderService.createOrder(buyer.getId(), purchaseItems));
+        assertThrows(CreateOrderException.class, () -> createOrder(buyer.getId(), purchaseItems));
     assertEquals("Insufficient balance", exception.getMessage());
 
     // product quantity should not be reduced
@@ -124,9 +117,7 @@ void shouldRejectOrderWithInsufficientProductQuantity() {
     purchaseItems.setPurchaseItem(product.getId(), 2);
 
     CreateOrderException exception =
-        assertThrows(
-            CreateOrderException.class,
-            () -> orderService.createOrder(buyer.getId(), purchaseItems));
+        assertThrows(CreateOrderException.class, () -> createOrder(buyer.getId(), purchaseItems));
     assertEquals("Insufficient stock for product: " + product.getId(), exception.getMessage());
 
     // buyer balance should not be reduced
@@ -145,7 +136,7 @@ void shouldNotThrowExceptionIfStockAndBuyerBalanceIsJustEnough() {
     PurchaseItems purchaseItems = new PurchaseItems();
     purchaseItems.setPurchaseItem(product.getId(), 1);
 
-    orderService.createOrder(buyer.getId(), purchaseItems); // should not throw exception
+    createOrder(buyer.getId(), purchaseItems); // should not throw exception
   }
 
   @Test
@@ -162,7 +153,7 @@ void shouldReduceProductQuantityAndBuyerBalanceWhenOrderIsSuccessful() {
     purchaseItems.setPurchaseItem(product1.getId(), 2);
     purchaseItems.setPurchaseItem(product2.getId(), 3);
 
-    orderService.createOrder(buyer.getId(), purchaseItems);
+    createOrder(buyer.getId(), purchaseItems);
 
     assertEquals(8, productRepository.findById(product1.getId()).orElseThrow().getQuantity());
     assertEquals(7, productRepository.findById(product2.getId()).orElseThrow().getQuantity());
@@ -192,7 +183,7 @@ void shouldIncreaseSellersBalance() {
     purchaseItems.setPurchaseItem(product1.getId(), 2);
     purchaseItems.setPurchaseItem(product2.getId(), 3);
 
-    orderService.createOrder(buyer.getId(), purchaseItems);
+    createOrder(buyer.getId(), purchaseItems);
 
     assertBigDecimalEquals(
         new BigDecimal(15), userRepository.findById(seller1.getId()).orElseThrow().getBalance());
@@ -213,7 +204,7 @@ void shouldCreateOrder() {
     purchaseItems.setPurchaseItem(product1.getId(), 2);
     purchaseItems.setPurchaseItem(product2.getId(), 5);
 
-    Order order = orderService.createOrder(buyer.getId(), purchaseItems);
+    Order order = createOrder(buyer.getId(), purchaseItems);
 
     assertEquals(buyer.getId(), order.getBuyerUserId());
     assertEquals(
@@ -234,12 +225,32 @@ void shouldEachCreatedOrderHasDifferentId() {
     PurchaseItems purchaseItems = new PurchaseItems();
     purchaseItems.setPurchaseItem(product.getId(), 1);
 
-    Order order1 = orderService.createOrder(buyer.getId(), purchaseItems);
-    Order order2 = orderService.createOrder(buyer.getId(), purchaseItems);
+    Order order1 = createOrder(buyer.getId(), purchaseItems, "request-01");
+    Order order2 = createOrder(buyer.getId(), purchaseItems, "request-02");
 
     assertNotEquals(order1.getId(), order2.getId());
   }
 
+  @Test
+  void shouldOneOrderBeingCreatedOnly_IfCreateOrderWithSameRequestIdTwice() {
+    User buyer = randomUsernameUserBuilder().balance(new BigDecimal(10)).build();
+    userRepository.save(buyer);
+
+    Product product = productBuilder().quantity(5).price(new BigDecimal(1)).build();
+    productRepository.save(product);
+
+    PurchaseItems purchaseItems = new PurchaseItems();
+    purchaseItems.setPurchaseItem(product.getId(), 1);
+
+    Order order1 = createOrder(buyer.getId(), purchaseItems, "request_id");
+    Order order2 = createOrder(buyer.getId(), purchaseItems, "request_id");
+
+    assertOrderEquals(order2, order1);
+    assertEquals(4, productRepository.findById(product.getId()).orElseThrow().getQuantity());
+    assertBigDecimalEquals(
+        new BigDecimal(9), userRepository.findById(buyer.getId()).orElseThrow().getBalance());
+  }
+
   @Test
   void shouldAutoRetry_WhenOneThreadMayFailJustDueToRacing() {
     User seller = randomUsernameUserBuilder().balance(new BigDecimal(999)).build();
@@ -254,8 +265,8 @@ void shouldAutoRetry_WhenOneThreadMayFailJustDueToRacing() {
     purchaseItems.setPurchaseItem(product.getId(), 1);
 
     // 2 threads try to buy the same product at the same time
-    Thread thread1 = new Thread(() -> orderService.createOrder(buyer.getId(), purchaseItems));
-    Thread thread2 = new Thread(() -> orderService.createOrder(buyer.getId(), purchaseItems));
+    Thread thread1 = new Thread(() -> createOrder(buyer.getId(), purchaseItems, "request-01"));
+    Thread thread2 = new Thread(() -> createOrder(buyer.getId(), purchaseItems, "request-02"));
 
     thread1.start();
     thread2.start();
@@ -270,11 +281,20 @@ void shouldAutoRetry_WhenOneThreadMayFailJustDueToRacing() {
     assertEquals(0, productRepository.findById(product.getId()).orElseThrow().getQuantity());
   }
 
+  private Order createOrder(String buyerUserId, PurchaseItems purchaseItems) {
+    return orderService.createOrder(buyerUserId, purchaseItems, "dummy_request_id");
+  }
+
+  private Order createOrder(String buyerUserId, PurchaseItems purchaseItems, String requestId) {
+    return orderService.createOrder(buyerUserId, purchaseItems, requestId);
+  }
+
   private void assertOrderEquals(Order expected, Order actual) {
     assertEquals(expected.getId(), actual.getId());
     assertEquals(expected.getBuyerUserId(), actual.getBuyerUserId());
     assertEquals(
         expected.getPurchaseItems().getProductIdToQuantity(),
         actual.getPurchaseItems().getProductIdToQuantity());
+    assertEquals(expected.getRequestId(), actual.getRequestId());
   }
 }