[GENERAL] "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@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@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 relationsSchema |         Name         |   Type   |  Owner
--------+----------------------+----------+----------public | test_serial          | table    | postgrespublic |
test_serial_uid_seq | sequence | postgrespublic | test_serial_uid_seq1 | sequence | postgres 
(3 rows)




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