Task.Joining numbers using Lists<string>

using System;
using System.Collections.Generic;
/*20 40 10 10 30 80
25 20 40 30 10
output 10 20 25 30 40 80*/
class JoinNumbers
{
    static void Main()
    {
        List<string> listOne = new List<string>(Console.ReadLine().Split(' '));
        List<string> listTwo = new List<string>(Console.ReadLine().Split(' '));

        List<string> joinNumbers = RemoveOccurNumbers(listOne);
        joinNumbers = UnionAllNumber(joinNumbers,listTwo);

        joinNumbers.Sort();
        Console.WriteLine("Now numbers are sorted!");
        foreach (string i in joinNumbers)
        {
            Console.Write(i + " ");
        }
        Console.WriteLine();
    }
    static List<string> UnionAllNumber(List<string> joinNumbers, List<string> listTwo)
    {
        foreach (string i in listTwo)
        {
            if(!joinNumbers.Contains(i))
            {
                joinNumbers.Add(i);
            }
        }
        return joinNumbers;
    }
    static List<string> RemoveOccurNumbers(List<string> listOne)
    {
        List<string> union = new List<string>();
        foreach (string item in listOne)
        {
            if(union.Contains(item))
            {
                continue;
            }
            union.Add(item);
        }
        return union;
    }
}