Improve error message for duplicate labels in enum types

Hi,

Currently, when creating an enum type, duplicate labels are caught by a unique
index on pg_enum, resulting in a generic error message.

postgres=# create type t as enum ('a','b','a');
ERROR: duplicate key value violates unique constraint "pg_enum_typid_label_index"
DETAIL: Key (enumtypid, enumlabel)=(16418, a) already exists.

I propose adding an explicit check for duplicate labels during enum creation,
so that a more user-friendly and descriptive error message can be produced,
similar to what is already done in ALTER TYPE ... ADD VALUE
or ALTER TYPE ... RENAME VALUE .. TO ....

With the attached patch applied, the error message becomes:

ERROR: label "a" used more than once

Regards,
Yugo Nagata

--
Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>