Duplicate caused by subform refresh

Hi,
I have a form with a subform and each are tied to a different table. I
have coding to refresh the calculations in my form based on updating
information in the subform and vice versa. When I update at certain times I
will get duplicate records created in the table connected to the subform. I
changed the field to a primary key but now I get an error that it will create
a duplicate. Does anyone have any ideas on how to stop this from happening?
--
Thank You,
Steve