rename constraint behavior for duplicate names?

I'm starting to get into PostgreSQL development by implementing:
%Allow ALTER TABLE ... ALTER CONSTRAINT ... RENAME

from the TODO. I've been looking through the code from CommentConstraint
and ATExecDropConstraint and they error out on duplicate constraint
names for a relation. However, ADD CONSTRAINT's code checks for
duplicates and errors out, so would the stuff in comment/drop be useless
checks then? And I would not have to worry about duplicate constraint
names for my rename code?

Allan Wang