Merging into one collection

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;

namespace Основы.Практика
{
    class Program
    {
        static void Main()
        {
            List<string> collection = new List<string>();

            string[] firstArray =
            {
                "2", "2", "2", "4", "5"
            };

            string[] secondArray =
            {
                "4", "4", "4", "2", "1"
            };

            Console.WriteLine("Перед сортировкой:");
            PrintArray(firstArray);
            PrintArray(secondArray);

            MergeUniqueArraysValuesIntoCollection(firstArray, secondArray, collection);

            Console.WriteLine("\nОбъеденная коллекция:");

            foreach (var item in collection)
            {
                Console.Write(item + " ");
            }
        }

        public static void PrintArray(string[] array)
        {
            foreach (var item in array)
            {
                Console.Write(item + " ");
            }

            Console.WriteLine();
        }

        public static void MergeUniqueArraysValuesIntoCollection(string[] array1, string[] array2, List<string> collection)
        {
            AddUniqueArrayValuesToCollection(array1, collection);
            AddUniqueArrayValuesToCollection(array2, collection);
        }

        public static void AddUniqueArrayValuesToCollection(string[] array, List<string> collection)
        {
            foreach (string item in array)
            {
                if (collection.Contains(item) == false)
                {
                    collection.Add(item);
                }
            }
        }
    }
}