BUG #14827: "ALTER TABLE... IF NOT EXISTS...ADD.. BIGSERIAL" leaves extra sequences

The following bug has been logged on the website:

Bug reference: 14827
Logged by: Hendrik Visage
Email address: hvisage(at)gmail(dot)com
PostgreSQL version: 9.6.5
Operating system: Debian 9
Description:

Goodday,
With an alter table to add an UID field if it doesn't exist, the process
creates a sequence, but when the column does exist, the created sequence,
that isn't used, is just left there, and subsequent runs keeps adding extra
sequences.

postgres(at)tracsdbhvt01:~$ cat test-serial.sql
create database test;
\c test
create table test_serial ( teststring varchar(5));
alter table test_serial add column if not exists uid BIGSERIAL;
alter table test_serial add column if not exists uid BIGSERIAL;
\d
\d test_serial
postgres(at)tracsdbhvt01:~$ psql -p 5433 < test-serial.sql
CREATE DATABASE
You are now connected to database "test" as user "postgres".
CREATE TABLE
ALTER TABLE
NOTICE: column "uid" of relation "test_serial" already exists, skipping
ALTER TABLE
List of relations
Schema | Name | Type | Owner
--------+----------------------+----------+----------
public | test_serial | table | postgres
public | test_serial_uid_seq | sequence | postgres
public | test_serial_uid_seq1 | sequence | postgres
(3 rows)

Table "public.test_serial"
Column | Type | Modifiers
------------+----------------------+-----------------------------------------------------------
teststring | character varying(5) |
uid | bigint | not null default
nextval('test_serial_uid_seq'::regclass)

postgres(at)tracsdbhvt01:~$