using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<string> originalList  = new List<string>()

        {

            @"AAA\BBB",

            @"AAA\CCC",

            @"AAA\CCC",

            @"BBB\XXX",

            @"BBB",

            @"BBB\XXX",

            @"BBB\XXX"

        };

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

        foreach(var g in originalList .GroupBy(x => x).Select(x => x.ToList()))

        {       

            for(int i = 0; i < g.Count; i++)

            {

                outputList.Add(string.Format("{0}[{1}]", g[i], i + 1));

            }

        }

        Console.WriteLine(string.Join("\n", outputList));

    }

}