How to avoid the repeated values in CheckBoxList

I have a TextBox, Button and CheckBoxList. If I click the checkbox and click the Button the value is displayed in TextBox. My TextBox is Autocomplete TextBox. if I select the value from auto complete that value in the CheckBoxList is removed. Here is my code:

     public void ListBox()
       {
            lstCheckBoxList.DataSource = s.sqlconn("select CityName from City");
            lstCheckBoxList.DataValueField = "CityName";
            lstCheckBoxList.DataBind();
       }

    protected void button1_Click(object sender, EventArgs e)
    {
         ListBox();

         foreach (ListItem li in lstCheckBoxList.Items)
         {
            if (li.Selected)
            {
                txtPracticeGroup.Text+= li.Text+ ";  ";
            }
         }
    }