Inner Join getting repeated value--Urgent Help needed

public void viewInvoiceByCompanyName(string cid, GridView viewingv)
    {
        try
        {
            DataSet ds = new DataSet();
            string connectionString = ConfigurationManager.ConnectionStrings["InvoiceConnectionString"].ConnectionString;
            string selectSql = "Select CustomerDetails.CompName,InvoiceParent.InvDate,InvoiceParent.InvoiceNo,InvoiceParent.CusId,InvoiceParent.PoNo,InvoiceParent.DoNumber,InvoiceParent.PayTerms,InvoiceParent.DueDate,InvoiceParent.ShipDate,InvoiceItem.ItemNo, InvoiceItem.Description,InvoiceItem.Qty,InvoiceItem.UnitPrice,InvoiceItem.TotalAmount,InvoiceItem.SubTotal,InvoiceItem.Gst,InvoiceItem.Total,InvoiceItem.Deposit,InvoiceItem.AmountDue FROM CustomerDetails INNER JOIN InvoiceItem ON CustomerDetails.CusId = InvoiceItem.CusId INNER JOIN InvoiceParent ON CustomerDetails.CusId = InvoiceParent.CusId WHERE InvoiceParent.CusId = @cusId";
            SqlConnection conn = new SqlConnection(connectionString);
            SqlDataAdapter da = new SqlDataAdapter(selectSql, conn);
            da.SelectCommand.Parameters.AddWithValue("@cusId", cid);
            da.Fill(ds, "Items");
            viewingv.DataSource = ds.Tables["Items"];
            viewingv.DataBind();

        }
        catch (Exception ex)
        {
            string msg = ex.Message;
        }


    }