Check loop ข้อมูลใน text file ถ้าข้อมูลที่กรอกลงใน textbox ซ้ำกับข้อมูลที่อยู่ใน loop ไม่ไห้เขียนลงtext file

using System;
using System.Linq;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Text.RegularExpressions;
using System.Threading;
using System.Diagnostics;

namespace CheckStock1
{
    public partial class SR : Form
    {
        public string A { get; set; }
        public string B { get; set; }
        public SR(string data,string data2)
        {
            InitializeComponent();
            A = data;
            B = data2; 
        }
        string log = "CheckStock.txt";
        private List<string> array = new List<string>();

        private void SR_Load(object sender, EventArgs e)
        {
            
                if (File.Exists(log) == false)
                {
                    f = new FileInfo(log);
                    StreamWriter sw = f.CreateText();
                    textBox1.Text = "";
                    sw.Close();
                }
                textBox1.Focus();
                textBox3.Text = "0";
                string line;
                StreamReader sr = new StreamReader(log);
                while ((line = sr.ReadLine()) != null)
                {
                    array.Add(line);
                }
                sr.Close();
            }
        private FileInfo f;
        private void button1_Click_1(object sender, EventArgs e)
        {
            Menu m1 = new Menu();
            m1.Show();
            this.Hide(); 
        }
        
        string C = DateTime.Now.ToString("yyyy-MM-dd");
        private void textBox1_KeyDown_1(object sender,KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Enter)
            {
                if (textBox1.Text.Length >= 15 || textBox1.Text.Length < 5)
                {
                    MessageBox.Show("Number Invalid");
                    textBox1.Text = "";
                    return;
                }
                string str = ((TextBox) sender).Text;
                if (!array.Contains(str))
                {
                    array.Add(str);
                    int Number = Convert.ToInt32(textBox3.Text.ToString());
                    string nb = (++Number).ToString();
                    textBox3.Text = nb;
                    textBox2.Text = textBox1.Text.Substring(0, 4);
                    label5.Text = textBox1.Text;
                    string log = "CheckStock.txt";
                    this.textBox1.Multiline = true;
                    if (File.Exists(log) == true)
                    {
                        f = new FileInfo(log);
                        StreamWriter sw = f.AppendText();
                        sw.WriteLine(C + "\t" + A + "\t" + B + "\t" + textBox1.Text + "\t" + textBox2.Text + "\t" + "1");
                        textBox1.Text = "";
                        sw.Close();
                    }
                }
                else
                {
                    MessageBox.Show(textBox1.Text + "   Serial Duplicate");
                    textBox1.Text = "";
                    return;
                }
            }
        }
      }
    }