using System;                   

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

         int identical = 0;

        int [] A = new int[6]  { 1,  2, 3, 5, 2, 5};

        Dictionary<int, KeyValuePair<int, int>> dic = new Dictionary<int, KeyValuePair<int, int>>(); /* A[i], the number of repetition of 3 so far (Rep[i]),

                    the number of identical pair so far ( Iden[i]). */

        for (int i = 0; i < A.Length; i++)

        {

            if (!dic.ContainsKey(A[i]))

                dic.Add(A[i], new KeyValuePair<int, int>(0,0));

            else

            { 

               KeyValuePair<int,int> valDic = dic[A[i]];

​

                KeyValuePair<int, int> newVal;

                if (valDic.Key < 1)

                    newVal = new KeyValuePair<int, int>(1, 1);

                else

                {

                    int preIdenticalPair = valDic.Value;

                    int preReptation = valDic.Key;

                    int newRepetation = ++preReptation;

                    int newIdenticalPair = preIdenticalPair + newRepetation;

                    newVal = new KeyValuePair<int, int>(newRepetation, newIdenticalPair);

                }

​

                dic[A[i]] = newVal;

            }

        }

​

        //summation of all identical pairs

        foreach (KeyValuePair<int, KeyValuePair<int, int>> pair in dic)

            identical += pair.Value.Value;

        Console.WriteLine(identical);

    }

}