Коллекции-5

using System;
using System.Collections.Generic;

namespace Homework5
{
    internal class Program
    {
        static void Main(string[] args)
        {
            string[] strings1 = { "1", "2", "1" };
            string[] strings2 = { "3", "2" };

            List<string> resultingList = new List<string>();

            resultingList.Add(strings1[0]);

            TryAddElement(strings1, resultingList);
            TryAddElement(strings2, resultingList);

            for (int i = 0; i < resultingList.Count; i++)
                Console.WriteLine(resultingList[i]);
        }

        private static void TryAddElement(string[] strings, List<string> resultingList)
        {
            for (int i = 0; i < strings.Length; i++)
            {
                bool alreadyExists = false;
                for (int j = 0; j < resultingList.Count; j++)
                {
                    if (strings[i] == resultingList[j])
                        alreadyExists = true;
                }

                if (alreadyExists == false)
                    resultingList.Add(strings[i]);
            }
        }
    }
}