Alex Ander

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

public class Program
{
	public static void Main()
	{
		var a = new[] { 1, 1, 2, 3, 3, 4, 4, 5 };
		var r = a.Where(x => a.Count(n => n == x) > 1).Distinct();
		
		foreach (var item in r)
		{
			Console.WriteLine(item);
		}
	}
}