How to run Update query in Ms access from Vb.net application

Hi There,
I have created an application using VB.net 2003 backend Ms access. On my Vb.net application I have a button and on the onclick event,  Im trying to run an Update query( UpdatePO)  (that updates data from one table to another table) that I have in my backend Ms access  database.

The problem is, it did run the update query the first time, but when I do it the next time, I get an ERROR
The changes you requested to the table were not successful becoz they would create duplicate values in the index, primary key or relationship. Change the data in the field that contain duplicate data,remove the index, or redefine to permit duplicate.

The same update query when I run from the ms access it has no problem any number of times I run it. It is not creating any duplicate as it's only an update, but for some reason my onclick event in Vb.net assumes it is going to create duplicate and not just update the existing data.
Im wondering if I some how have to make the code in my Vb.net understand to continueupdate on error.
Please find my onclick event below


Private Sub IMPBTN_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles IMPBTN.Click
       
        Try

            Dim conn As System.Data.OleDb.OleDbConnection

            Dim strConn As String

            strConn = "Provider = Microsoft.Jet.OLEDB.4.0;"

            strConn &= "Data Source = F:\Rail.mdb;"

            conn = New System.Data.OleDb.OleDbConnection(strConn)



            Dim cmd As New System.Data.OleDb.OleDbCommand("UpdatePO", conn)

            cmd.CommandType = CommandType.StoredProcedure
                        conn.Open()

            cmd.ExecuteNonQuery()

            conn.Close()

        Catch ex As Exception
            MsgBox(ex.Message + vbCrLf + ex.Source & vbCrLf & vbCrLf & ex.StackTrace, MsgBoxStyle.Critical, "Error")
        End Try
    End Sub


How can I make it work .I will appreciate any help on this.
Thanks
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Private Sub IMPBTN_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles IMPBTN.Click
        ' start1()
        Try
 
            Dim conn As System.Data.OleDb.OleDbConnection
 
            Dim strConn As String
 
            strConn = "Provider = Microsoft.Jet.OLEDB.4.0;"
 
            strConn &= "Data Source = F:\Distribution & Logistics\Railcars\RailTruck Program\Rail.mdb;"
 
            conn = New System.Data.OleDb.OleDbConnection(strConn)
 
 
 
            Dim cmd As New System.Data.OleDb.OleDbCommand("UpdatePO", conn)
 
            cmd.CommandType = CommandType.StoredProcedure
           
            conn.Open()
 
            cmd.ExecuteNonQuery()
 
            conn.Close()
 
 
 
 
 
        Catch ex As Exception
                       MsgBox(ex.Message + vbCrLf + ex.Source & vbCrLf & vbCrLf & ex.StackTrace, MsgBoxStyle.Critical, "Error")
        End Try
    End Sub