How to Gridview Using Dropdown Value more time Display so that value distinct

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C#

SQL-Server

My Source Wise Gridview Dropdown More Time Display How to Solve this Problem,

My Source is:

private void LoadAdjustLeave()
    {
        SqlCommand cmd = new SqlCommand();
        SqlDataAdapter da;
        cmd.Connection = con;
        cmd.CommandType = CommandType.Text;
        try
        {
            //string sql = " select USER_ID as UserID, LateDays,Leave,Remainingminutes,Month,Year from Accounts_LateEntry_Salary_Deduct where User_ID='" + gvStaffDetails.SelectedRow.Cells[2].Text.ToString().Trim() + "'";
            string sql = "select convert(VARCHAR(12),LeaveDate) as leavedate, InformType, TotalDays, Leave,Applyhrs from deletetable where UserID='" + gvStaffDetails.SelectedRow.Cells[2].Text.ToString().Trim() + "' and Month='" + Session["paymonth"] + "' and Year='" + Session["payyear"] + "'";
            da = new SqlDataAdapter(sql, con);
            //DataSet ds = new DataSet();
            DataTable dt = new DataTable();
            da.Fill(dt);
            con.Open();
            gvadjustleave.DataSource = dt;
            Session["AdjustLeave"] = dt;
            gvadjustleave.DataBind();
            con.Close();
            
        }
        catch (Exception Ex1)
        {
            LblErr.Text = Ex1.Message.ToString();
            return;
        }
    }

load time Load Data Multiple time Show Dropdown,so i don't want Below Example.
InformType
Not Informed
Not Informed
Prior
Prior
Prior
Prior
Late informed
Not Informed
Not Informed
Not Informed
Not Informed
Not Informed
Not Informed
Not Informed

So I don't want Dropdownlist (above) Result

I want Dropdown List load Data Result below exaple

InformType
Prior
Late informed
Not Informed


Thanks