Duplicating records with portals

I am working with Mac system 10.4 and filemaker Pro vers 9. I have an invoice file that includes a portal with ten rows for line items. A relationship exxists to a line items file and everything functions fine.
However, my customer often needs to duplicate invoice records and this is very important to his business. But when a record is duplicated using usual methods, the crucial line items information in the portal is not duplicated. (Duping line items within the portal within the same record is no problem - but nothing happens in the newly created record.)
My proficiency level is probably between beginner and intermediate but I have not been able to solve this problem for days. Does anyone out there have a script for me on this. I will certainly appreciate the help. ;-)
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