My textBox leave triggers twice? why it is ...(winform)

private void txtAccCentre_Leave(object sender, EventArgs e)
       {

           if (txtAccCentre.Text.Trim() != "")
           {

               if (txtAccCentre.Text.Length >= 3)
               {
                   txtCurrency.Text = string.Empty;
                   cbCurrency.DataSource = null;
                   txtAccCentre.LostFocus += new EventHandler(txtAccCentre_LostFocus);
               }
               else
               {
                   string result = CustomMessageBox.ShowBox("Filter text length should be greater than or equal to 3", "Error", "");
                   if (txtAccCentre.Text.Length == 0)
                   {
                       txtAccCentre.Text = "Code/Desc";
                       txtAccCentre.ForeColor = SystemColors.GrayText;
                   }

               }
           }
       }

txtAccCentre.LostFocus += new EventHandler(txtAccCentre_LostFocus);