using System.Collections.Generic;

using System;

public class Program

{

    Dictionary<string,int> l=new Dictionary<string,int>();

    public Dictionary<string,int> dup(String[] f)

    {

    foreach(var item in f)

    {

        if(l.ContainsKey(item)){

            l[item]++;

    }

        else

        {

        l.Add(item,1);

        }

    }

        return l;

    }

    public static void Main()

    {

        Program p=new Program();

        string[] a={"ab","av","ab","ad"};

        Dictionary<string,int> d=p.dup(a);

        foreach(var item in d)

        {

            Console.WriteLine(item.Key);

        }

    }

}

​