Many times in my work I'm sent a spreadsheet with a "Please load to production" note. The problem is that frequently they don't check all of the data and we end up trying to add duplicates into the production files. Now while it's possible to run an INSERT INTO ..... WHERE NOT EXISTS deal, in some cases we need to run this multiple times until the records are loaded. And there's no early warning you're just discarding records arbitrarily.
I've begun to use the following SQL to find duplicate entries, prior to attempting to add the records, and I can even go back to the spreadsheet - highlight the duplicates and ask which is correct!
with stuff as (select rrn(mf) as recno, mf.* from mylib/myfile mf) select * from stuff a left join stuff b on a.f1 = b.f1 [and a.f2 = b.f2....] where a.recno > b.recno
just remember to join on all of the primary keys.