et_FORM_DATA_ADD event

I have performed certain calculations when a record is added in A/R invoice.I have called the function on the et_FORM_DATA_ADD event as,

If BusinessObjectInfo.EventType = SAPbouiCOM.BoEventTypes.et_FORM_DATA_ADD And (BusinessObjectInfo.FormTypeEx = "133") And BusinessObjectInfo.ActionSuccess = True Then
 SaveStockTrans()
End If

Usually the event fires once but sometimes it fires twice.I can not predict when it fires twice.