transaction serial number overwrite each other?

[Tengkorak]

I have code to generate transaction number.
hire my code :

Function TransNumber() As String
        Dim oCmd As New OleDb.OleDbCommand
        Dim sNo As String = "P"
 
        Try
            oCmd.CommandText = "select top 1 pono from PURCHASE where order by pono  desc"
            Dim dr As OleDb.OleDbDataReader
            dr = oCmd.ExecuteReader
            If dr.Read Then
                sNo = dr.Item("pono") + 1
            Else
                sNo += "001"
            End If
            dr.Close()
        Catch ex As Exception
 
        End Try
    End Function

I had a problem when there are multiple users using the transaction or the serial number is, because the data overwrite each other.

how to make the correct serial number?