Cannot have multiple items selected in a DropDownList in asp.net

Question

I have several DropdownList, however there are two on which I am using SelectedIndexChange event. I am trying to get data from Database and fill respective DropdownList with values. Earlier I had an issue where both Dropdownlist gets same value, whichever code is written later. Then somehow that was fixed then I am getting this error .

Cannot have multiple items selected in a DropDownList.

Heres how I am getting data from database :

string sqlqry = "select * from Newenrollee where enrollecode=@enrollecode ";
//SqlCommand cmd = new SqlCommand(sqlqry, conn);
using (var cmd = new SqlCommand(sqlqry, conn))
{
    cmd.Parameters.AddWithValue("@enrollecode", txtenrolleecode.Text);
    dt.Rows.Clear();
    da1 = new SqlDataAdapter(cmd);
    da1.Fill(dt);
    if (dt.Rows.Count > 0)
    {
        string regdate = (Convert.ToDateTime(dt.Rows[0]["dateofappointment"])).ToString();
        DDLAPDay.Text = regdate.Substring(0, 2);
        int mnthAD = (Convert.ToDateTime(dt.Rows[0]["dateofappointment"])).Month;
        string mnthAD1 = "";
        if (mnthAD > 0 && mnthAD < 10)
            mnthAD1 = "0" + mnthAD.ToString();
        else
            mnthAD1 = mnthAD.ToString();

        DDLADMonth.Text = mnthAD1;
        DDLADYear.ClearSelection();

        DDLADYear.Text = (Convert.ToDateTime(dt.Rows[0]["dateofappointment"])).Year.ToString();
        DDLDay.Text = DOB.Text.Substring(0, 2);
        int daydob = (Convert.ToDateTime(dt.Rows[0]["DOB"])).Month;
        string daydob1 = "";
        if (daydob > 0 && daydob < 10)
            daydob1 = "0" + daydob.ToString();
        else
            daydob1 = daydob.ToString();
        DDLMonth.Text = daydob1.ToString();
        DDLYear.ClearSelection();
        DDLYear.Text = (Convert.ToDateTime(dt.Rows[0]["DOB"])).Year.ToString();
    }
}

These are the Dropdownlist that I am using. Let me know what is wrong. Thanks.