Check if object exists in Generic List of class objects in C# .Net

i have a collections class of customers and in my register page i am adding a customer to the collection i have a List<Customer> and i want to be able to add a customer only if the Username is not the same and if it is the same i dont want to be able to add to the list how can i do this? register class

        customer = new PreferredCustomer(firstname.Text, lastname.Text, birthdate.Text,
            address.Text, city.Text, state.Text, zipcode.Text, phonenumber.Text,
            login.Text, password.Text, commentBox.Text);
        Session["theCustomer"] = customer;
       
        Collections.PreferredCustomers.Add(customer);