Duplicate Insertion in DB

I have a GridViewwith rows containing .net checkboxcontrol. User would check the rows and upon clicking the button it would insert the checked rows in the table GeneratedInvoiceBatches

VB Code:

Protected Sub btnGenerate_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnGenerate.Click
            Dim grItem As DataGridItem
            Dim i As Integer = 0


            For Each grItem In grdTransactions.Items

                If grItem.ItemType = ListItemType.AlternatingItem Or grItem.ItemType = ListItemType.Item Then

                    If CType(grItem.Cells(0).FindControl("chkSelect"), CheckBox).Checked = True Then

                        Utilities.InsertUnsettledBatchesForInvoice(grItem.Cells(5).Text.Trim, grItem.Cells(7).Text.Trim, grItem.Cells(11).Text.Trim)

                        i = i + 1

                    End If

                End If

            Next

            If i = 0 Then
                Response.Write("<script> alert('No selected record(s) to process!');</script>")
            Else

                Response.Write("<script> alert('Operation Successful!');</script>")
                BindDataGrid()
            End If

        End Sub

Stored Procedure:

Create proc [dbo].[spInsertUnsettledBatchesForInvoice]

@TerminalID varchar(8),
@BatchNumber int,
@SettledAmount money

as


insert into GeneratedInvoiceBatches (GenerationDate,TerminalID,BatchNumber,SettledAmount,isPayment,isHSBCFile,paymentdate)
values
(GetDate(),@TerminalID,@BatchNumber,@SettledAmount,1,1,getdate())

Problem is sometimes it inserts duplicate rows say once in three months. Can somebody identify the issue? Is it something to do with the checkbox control? How can I avoid this duplication?