Basic locking question

I'm not very familiar with explicitly locking things in SQL, so this may
be a dumb question, but.....

I've got a stored proc that takes a text key and returns an ID. If the
key isn't in a lookup table, it adds it and will return the
auto-generated serial number. If it's already there, it simply returns
the existing serial number.

I'm concerned about multiple sessions calling this at the same time with
the same key. While I *could* just put in a unique constraint and let
things fail, that's hardly graceful. It seems I should be able to lock
the lookup table, but I don't really know what mode to lock that table
in. Will ROW EXCLUSIVE keep the same key from being entered twice?