правочка

CarRepairShop/CarRepairShopDatabaseImplement/Implements/MessageInfoStorage.cs

@@ -38,7 +38,7 @@ namespace CarRepairShopDatabaseImplement.Implements
             using var context = new CarRepairShopDatabase();
             model.ClientId = context.Clients.FirstOrDefault(x => x.Email.Equals(model.SenderName))?.Id;
             var message = MessageInfo.Create(model);
-            if(message == null) return null;
+            if(message == null || context.Messages.Any(x => x.MessageId.Equals(model.MessageId))) return null;
             context.Messages.Add(message);
             context.SaveChanges();
             return message.GetViewModel;