fix: prevent duplicate delivery assignments

backend/src/test/java/com/linhelp/delivery/DeliveryAssignmentServiceTests.java

@@ -69,6 +69,18 @@ class DeliveryAssignmentServiceTests {
                 .hasMessageContaining("自提订单无需派单");
     }
 
+    @Test
+    void cannotAssignSameGoodsOrderTwice() {
+        TestFixture fixture = new TestFixture();
+        GoodsOrderResponse order = fixture.preparedDeliveryOrder();
+        DeliveryUserResponse rider = fixture.rider();
+
+        fixture.assignmentService.assignGoodsOrder(order.getId(), rider.getId());
+
+        assertThatThrownBy(() -> fixture.assignmentService.assignGoodsOrder(order.getId(), rider.getId()))
+                .hasMessageContaining("订单已派单");
+    }
+
     private static class TestFixture {
         private final ProductService productService = new ProductService();
         private final GoodsOrderService goodsOrderService = new GoodsOrderService(productService);