remove database dulicate value

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C#

Visual-Studio

WinForms

Dear All,
how to remove the duplicate value when its select item from the database. this code is working fine but its show duplicate value in the combobo.
like: gold, gold, gold, silver, silver
its show 3 and 2 times in the combobox. so how to remove this error.
it is show only one - one times.

private void cmboItemType()    //for combobox 
        {
            string connstr = @"Server=.\SQLEXPRESS ;Initial Catalog=RPSJDB;Integrated Security=True; Max Pool Size=100";
            SqlConnection conn = new SqlConnection(connstr);
            SqlCommand cmd = new SqlCommand("SELECT cmbItemType FROM ItemTable", conn);
            conn.Open();
            SqlDataReader sdr = cmd.ExecuteReader();
            ArrayList ItemStore = new ArrayList();
            
            while (sdr.Read())
            {
                ItemStore.Add(new AddValue(sdr.GetString(0)));
            }
            sdr.Close();
            conn.Close();
            cmbItemType.DataSource = ItemStore;
            cmbItemType.DisplayMember = "Display";
            
        }
        public class AddValue
        {
            private string Displaym;
         
            public AddValue(string Display)
            {
                Displaym = Display;
            }
            public string Display
            {
                get { return Displaym; }
            }
        }