using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var contractNr = 123456789;

        Random random = new Random();

        var length = 3;

        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

        var chars2 = DateTime.Now.Year.ToString().Substring(3, 1) + DateTime.Now.DayOfYear;

        Console.WriteLine(chars2);

        List<string> results = new List<string>();

        for (var i = 0; i < 999; i++)

        {

            var result = new string (Enumerable.Repeat(chars, length).Select(s => s[random.Next(s.Length)]).ToArray());

            results.Add(result);

            Console.WriteLine(result);

        }

​

        var anyDuplicate = results.GroupBy()(x => x).Any(g => g.Count() > 1);

        Console.WriteLine(anyDuplicate);

    }

}