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

public class Test
{
	public static void Main()
	{
	      	Dictionary<int, string> d = new Dictionary<int, string>();
            d.Add(1000, "F1");
            d.Add(1001, "F2");
            d.Add(1002, "F1");
            d.Add(1003, "F4");
            d.Add(1004, "F2");

            var dublicate = d.ToLookup(x => x.Value, x => x.Key).Where(x => x.Count() > 1);

            foreach (var i in dublicate)
            {
                Console.WriteLine(i.Key);
            }
	}
}