using System;
using System.Collections.Generic;

public class Test
{
	public static void Main()
	{
        Console.WriteLine("Output {0}", returnSecondDuplicate(new[] { 'x', 'y', 'z', 'x', 'y' }));
        Console.WriteLine("Output {0}",  returnSecondDuplicate(new[] { 'a', 'a', 'b', 'a', 'c', 'b', 'a', 'c', 'b' }));
        Console.WriteLine("Output {0}", returnSecondDuplicate(new[] { 'a', 'a', 'a', 'b', 'a', 'c', 'b', 'a', 'c', 'b' }));
        Console.ReadLine();
	}
	
	public static char returnSecondDuplicate(char[] arr)
	{
		if (arr.Length == 0)
		    throw new ArgumentNullException("Empty Array passed");
		var dictionary = new Dictionary<char, int>();
		var duplicates = new List<char>();
		Char second = '\0';
		int i = 0;
		
        while (duplicates.Count != 2 && dictionary.Count != arr.Length)
        {
            if (!dictionary.ContainsKey(arr[i]))
                dictionary.Add(arr[i], 1);
            else if (!duplicates.Contains(arr[i]))
                duplicates.Add(arr[i]); // only add duplicates once (ignoring any duplicate duplicates!)

            second = duplicates.Count == 2 ? arr[i] : second;
            i++;
        }

        return second;
	}
}