BUG #5988: CTINE duplicates constraints

The following bug has been logged online:

Bug reference: 5988
Logged by: Marko Tiikkaja
Email address: marko(dot)tiikkaja(at)2ndquadrant(dot)com
PostgreSQL version: git master
Operating system: Linux
Description: CTINE duplicates constraints
Details:

CREATE TABLE IF NOT EXISTS duplicates some constraints if the new table
isn't created:

=# create table foo(a int primary key);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "foo_pkey"
for table "foo"
CREATE TABLE

=# create table if not exists foo(a int primary key);
NOTICE: relation "foo" already exists, skipping
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "foo_pkey1"
for table "foo"
CREATE TABLE

=# \d foo
Table "public.foo"
Column | Type | Modifiers
--------+---------+-----------
a | integer | not null
Indexes:
"foo_pkey" PRIMARY KEY, btree (a)
"foo_pkey1" PRIMARY KEY, btree (a)

It seems to do that at least for PRIMARY KEY and UNIQUE constraints.