//定义四个数组
    ArrayList number = new ArrayList();

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            string Path = Server.MapPath("info.txt");
            getStr(Path);

            for (int i = 0; i < number.Count; i++)
            {
                Response.Write(number[i].ToString()+" | ");
                //这里将所有的最后一列数据取出来了，然后判断是否有重复应该很简单了
                
            }
        }
    }

    //读取txt行中的每一个数据
    public void getStr(string Path)
    {
        string strLine = "";
        int i = 0;
        try
        {
            StreamReader sr = new StreamReader(Path, Encoding.GetEncoding("GB2312"));
            while ((strLine=sr.ReadLine()) != null)
            {
                string[] val = strLine.Split(' ');
                //将数据保存在ArrayList集合中
                number.Add(val[3]);
                Response.Write("当前第"+i+"行:<br/>");
                foreach (string s in val)
                {
                    Response.Write(s + "<br>");
                }
                i++;
                Response.Write("<hr>");
            }
            sr.Dispose();
            sr.Close();
        }
        catch
        {
        }
    }

------解决方案--------------------

探讨

string[] temp = File.ReadAllLines(@"C:\1.txt", Encoding.GetEncoding("GB2312"));
  temp = temp.ToList().Distinct().ToArray();
  File.WriteAllLines(@"C:\1.txt", temp);

例如数据是这样：
-------------------……