eCommerce Gateway - POI Import - Avoiding dupes

I currently import my PO's and they create our sales orders. I have been doing this for several years. The issue is that sometimes a PO is

received and it has already been used in our system. I have read that by modifying the ORIG_SYS_DOCUMENT_REF so that it is different

than the PO I can avoid the duplicate issue.

So I am trying to modify my EDI data file, my issue I am having is that i'm unable to find the correct data fields to populate so that the default

does not cause the field ORIG_SYS_DOCUMENT_REF to automatically populate with the PO number.