"alter table...if exists... add bigserial "still adds extra sequence

Good day,

See the sequence below, Postgresql 9.6.5 on Debian using the postgresql repository.

Question: Is this expected behaviour?

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

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)