using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var list1 = new[] { 1, 1, 2 };

        var list2 = new[] { 1, 2, 3 };

        var unioned = list1.Union(list2);

        var distinct = list1.Concat(list2).Distinct();

​

        Console.WriteLine("unioned:  " + string.Join(",", unioned.Select(x => x.ToString()).ToArray()));

        Console.WriteLine("distinct: " + string.Join(",", distinct.Select(x => x.ToString()).ToArray()));

    }

}