Find and return first duplicate in a generic list.

FindFirstDuplicate.cs

using System;
using System.Collections.Generic;
using System.Collections;

class FindFirstDuplicate
{
	public static void Main()
	{
		var input = new List<string> { "Foo", "Bar", "Bar", "Foo" };
		object result = GetFirstDuplicateValue(input);
		PrintDuplicateResult(result);

		var input2 = new List<int> { 0, 1, 2, 3, 12, 11, 112 };
		object result2 = GetFirstDuplicateValue(input2);
		PrintDuplicateResult(result2);

		Console.ReadKey();
	}

	public static object GetFirstDuplicateValue<T>(List<T> input)
	{
		var hash = new Hashtable();

		foreach (object listItem in input)
		{
			if (hash.ContainsKey(listItem))
			{
				var value = (Int32)hash[listItem];
				hash[listItem] = value + 1;
			}
			else
				hash.Add(listItem, 1);
		}

		foreach (object listItem in input)
			if ((Int32)hash[listItem] > 1)
				return (listItem);

		return null;
	}

	public static void PrintDuplicateResult(object input)
	{
		Console.WriteLine((input != null) ? input.ToString() : "No Duplicates.");
	}
}