Cannot add an entity with a key that is already in use Error

var Nwind_dc = new DataClasses1DataContext();
          
            foreach (var Cust in GridTable)
            {
                var customer = new Customer
                {
                    CustomerID = Cust.CustomerID,
                    CompanyName = Cust.CompanyName,
                    ContactTitle = Cust.ContactTitle


                };

                var order = new Order { OrderDate = Cust.OrderDate, EmployeeID = Cust.EmployeeID };
               
                customer.Orders.Add(order);
                Nwind_dc.Customers.InsertOnSubmit(customer);
                
             
            }
            Nwind_dc.SubmitChanges();