Its funny, I am reading through the forums and someone asks "Can I please have a REPLACE INTO, instead an INSERT INTO?" and I remember how often I have used a sed script to modify MySQL dumps to have exactly that.
So, use --replace and instead of getting INSERT INTO you will INSTEAD get REPLACE INTO. Buyer beward, REPLACE is a MySQL SQL, so you will not end up with a dump that can be used for other databases.

Though I hear you could just use a sed line to modify it back :)