弄个防止listBox重复添加Items的小东西都脑大啊该如何解决

弄个防止listBox重复添加Items的小东西都脑大啊
依然重复添加了，根本没办法防止重复的items ， 了个去啊。

C# code

        private void button2_Click(object sender, EventArgs e)        {            IPHostEntry iphost = Dns.GetHostEntry(textBox1.Text);                    //string ipaddress = ip.AddressFamily.ToString();                    //listBox1.Items.Add(ipaddress);                 foreach (IPAddress ip in iphost.AddressList)                  {                     if(!listBox1.SelectedItems.Contains(ip.ToString()))                     {                         listBox1.Items.Add(ip.ToString());                     }                     else                     {                         MessageBox.Show("Item Exists!");                        }                  }                }

C# code

private void button2_Click(object sender, EventArgs e){       IPHostEntry iphost = Dns.GetHostEntry(textBox1.Text);        foreach (IPAddress ip in iphost.AddressList)        {             if(!listBox1.Items.Contains(ip.ToString()))              {                   listBox1.Items.Add(ip.ToString());              }               else               {                     MessageBox.Show("Item Exists!");                   }      }        }