Hi,
TransactionNotification is one of the two SP given by SAP that you can modify to fulfill your needs (basically do some controls). You can search on the SCN, you'll find some references about it.
In your case, the control to add is the following:
IF@object_type='24'AND@transaction_type='A'
BEGIN
DECLARE@CheckNumint
SELECT@CheckNum=R1.CheckNum
FROMRCT1R1
INNERJOINORCTR0ONR1.DocNum=R0.DocEntry
WHERER0.DocEntry=@list_of_cols_val_tab_del
GROUPBYR1.CheckNum,R0.CardCode
HAVINGCOUNT(*)> 0
IFISNULL(@CheckNum, 0)<> 0
BEGIN
SELECT@error= 1,@error_message='Check number for this customer has already been entered'
END
END
Regards,
Eric