BUG #14940: Duplicated records inspite of primary key and uniqueconstraint

The following bug has been logged on the website:

Bug reference:      14940
Logged by:          sergey frolov
Email address:      sergey.frolov@smetarik.ru
PostgreSQL version: 9.6.6
Operating system:   Windows 10, 64
Description:

Hi, I have noticed duplicated records inspite of primary key and unique
constraint.  

select version ()
PostgreSQL 9.6.6, compiled by Visual C++ build 1800, 64-bit

The DDL is 
CREATE TABLE nb.nb_basedtl
(
  id integer NOT NULL, 
  base_id integer NOT NULL, 
  norm_id integer NOT NULL, 
  ...
  CONSTRAINT pk_nb_basedtl PRIMARY KEY (id),
  CONSTRAINT unq_nb_basedtl UNIQUE (norm_id, base_id),
  ....

The problem is

select ctid,xmin,xmax, id, base_id, norm_id from nb.nb_basedtl where id =
11658502 ;-- expected ONE row 

(0,49);364507;0;11658502;269;46203
(0,49);370881;0;11658502;269;46203
(0,49);370882;0;11658502;269;46203


select (select count(1) from nb.nb_basedtl), (select count(1) from (select
id, count(1) from nb.nb_basedtl  group by id having count(1) > 1 ) t )
3586895;50