Adding records to a related file

[Nicholas Geti]

How do I add additional records to a child (related) table from a master table. 
These tables are related on the Invoice Number field in each table and I can 
add one record just by doing SetField() on the child's invoice field but if I 
issue the command again it simply writes over the previous one as expected. How 
do I force creating a second record having the same invoice number?