Removing duplicate items from a listbox

private void button1_Click(object sender, EventArgs e)
{
     string strTmp = textBox1.Text;
    
     if (strTmp == string.Empty)
     {
          MessageBox.Show("I don't do empty strings", "Missing String");
          textBox1.Focus();
          return;
     }

     for (int i = 0; i < listBox1.Items.Count; i++)
     {
          if(listBox1.Items[i].ToString() == strTmp)
          {
               MessageBox.Show(strTmp + " is already in the listbox.", "Dupe");
               textBox1.Focus();
               return;
          }
     }
    
     // won't get here if there is a duplicate or empty string
     listBox1.Items.Add(textBox1.Text);
     textBox1.Text = string.Empty;
     textBox1.Focus();
}