BPM starting question

Question

Hi all,
I have developed a BPM which basically reads 2 files (INVOICES and BILLING) makes some transformation and send it to a SAP R/3 system.
Both files have a data field which it is used to correlate them.
So when sender system send INVOICES file or BILLING file, SAP XI read it, begin the BPM which waits for the other to come. When the other file come the BPM continue in their next steps.
But we have experienced the next situation:
- Sender systems send 2 INVOICES files (duplicate files) and some time later send a BILLING file (only one).
The process:
1.- SAP XI reads the first INVOICE file and starts a BPM process.
2.- SAP XI reads the other INVOICE file and starts another BPM process.
3.- SAP XI reads the BILLING file and enroute it to the 2 BPM process
Is there any configuration in my BPM or configuration scenario which doesn`t allow this situation. I only want one BPM for one pair of files.  
Thanks.

Answer 1

Thanks for your comments Udo.
I need the correlation because there are days in which I receive several invoices and billing files (specially the first days of the month due to the bussiness process in sender system).
But the source of the problem is on the sender system. Some times the generation of the files is done manually, and there are situations in which operator makes mistakes and generate multiple files for one day.
The onwer of receiver systems wants that SAP PI can control this kind of situations. One situation is what I have described before: the BPM collecting pattern should work with two files.
The normal behaviour is:
INVOICE1 --> BPM (instance #1)
BILLING1  --> BPM (instance #1)
INVOICE2 --> BPM (instance #2)
BILLING2  --> BPM (instance #2)
INVOICE3 --> BPM (instance #3)
BILLING3  --> BPM (instance #3)
INVOICE4 --> BPM (instance #4)
BILLING4  --> BPM (instance #4)
But my situation is:
INVOICE1 --> BPM (instance #1)
INVOICE1 --> BPM (instance #2)
BILLING1  --> BPM (instance #1) BPM (instance #2)
Regards.