Please apologize me if i am wrong.
According to your below statement, the duplicates are removed after only the reformat fields are executed. If so we can give "SUM FIELDS=NONE".
It will do the same..!
I thought that using the "DUPKEYS FIELDS=NONE" statement will remove the duplicates in any of 2 files before matching.
I think you have little bit more now.
Thanks and Regards,