IF SORTED - find unpairedin list Problem set 1

    // You've got an array of ints 2,2,3,3,4,5,5... Find an element without a pair.
    class Program
    {
        static void Main(string[] args)
        {
            var array = new int[] {2, 2, 3, 3, 4, 5, 5, 6, 9};
            var array2 = new int[array.Length];

            for (int index = 0; index < array.Length; index++)
            {
                var a = array[index];
                var nxtIndex = index + 1;

                if (nxtIndex == array.Length)
                {
                    Console.WriteLine(a);
                    break;
                }

                var b  = array[nxtIndex];

                if (b != a)
                {
                    Console.WriteLine(a);
                }
                else
                {
                    index++;
                }
            }

            Console.ReadKey();
        }
    }