Can COPY update or skip existing records?

Hey all,

I've got a table with a unique constraint across a few fields which I
need to regularly import a batch of data into. Is there a way to do it
with COPY without getting conflicts on the unique contraint? I have no
was of being certain that some of the data I'm trying to load isn't in
the table already.

Ideally I'd like it to operate like MySQL's on_duplicate_key_update
option, but for now I'll suffice with just ignoring existing rows and
proceeding with everything else.

Thanks,

-- 
Glenn