using System;

using System.Collections.Generic;

using System.Text;

public class FindDuplicates

{

    public string GetDuplicates(string value, string compareValue)

    {

        var dups = new HashSet<char>();

        var output = new StringBuilder();

        // Create a map of chars in first value

        var map = new HashSet<char>();

        var chars = value.ToCharArray();

        foreach (var c in chars)

        {

            if (!map.Contains(c))

            {

                map.Add(c);

            }

        }

        var compareChars = compareValue.ToCharArray();

        foreach (var c in compareChars)

        {

            if (map.Contains(c) && !dups.Contains(c))

            {

                dups.Add(c);

                output.Append(c);

            }

        }

        return output.ToString();

    }

    public static void Main()

    {

        var string1 = "ABCDGH";

        var string2 = "AEDFHR";

        var overlapFinder = new FindDuplicates();

        Console.WriteLine(overlapFinder.GetDuplicates(string1, string2));

    }

}