using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var tickets = new List<Ticket> {

          new Ticket {TicketNumber = "1"},

          new Ticket {TicketNumber = "2"},

          new Ticket {TicketNumber = "3"},

          new Ticket {TicketNumber = "4"},

          new Ticket {TicketNumber = "2"},

          new Ticket {TicketNumber = "1"}

       };

        UpdateTickets(tickets);

        foreach(var ticket in tickets.Where(x => x.IsUnique)) {

            Console.WriteLine(ticket.TicketNumber);

        }

    }

    public static void UpdateTickets(IList<Ticket> tickets)

    {

        var uniqueTickets = tickets.GroupBy (t => t.TicketNumber)

            .Where (t => t.Count () == 1)

            .SelectMany (t => t);

​

        foreach (var ticket in uniqueTickets)

        {

            ticket.IsUnique = true;

        }

    }

}

​

​

public class Ticket

{

    public string TicketNumber { get; set; }

    public bool IsUnique { get; set; }

}