The core CREATE FUNCTION code only enforces that IN parameter names are
non-duplicate, and that OUT parameter names are separately non-duplicate.
This is because some function languages might not have any confusion
between the two.  But in plpgsql, such names are all in the same namespace,
so we'd better disallow it.

Per a recent complaint from Dan S.  Not back-patching since this is a small
issue and the change could cause unexpected failures if we started to
enforce it in a minor release.