BUG #13760: order by . offset .. limit bug? when order by column has same value

The following bug has been logged on the website:

Bug reference:      13760
Logged by:          digoal
Email address:      digoal@126.com
PostgreSQL version: 9.4.5
Operating system:   CentOS 6.x x64
Description:

when order by column has same values, there will return (0,1) all times
whatever offset x.

postgres=# create table t(id int);
CREATE TABLE
postgres=# insert into t select 1 from generate_series(1,500);
INSERT 0 500
postgres=# select row_number() over(),ctid,* from t order by id desc offset
0 limit 5;
 row_number | ctid  | id
------------+-------+----
          2 | (0,2) |  1
          3 | (0,3) |  1
          4 | (0,4) |  1
          5 | (0,5) |  1
          1 | (0,1) |  1
(5 rows)

postgres=# select row_number() over(),ctid,* from t order by id desc offset
1 limit 5;
 row_number | ctid  | id
------------+-------+----
          3 | (0,3) |  1
          4 | (0,4) |  1
          5 | (0,5) |  1
          6 | (0,6) |  1
          1 | (0,1) |  1
(5 rows)

postgres=# select row_number() over(),ctid,* from t order by id desc offset
100 limit 5;
 row_number |  ctid   | id
------------+---------+----
        102 | (0,102) |  1
        103 | (0,103) |  1
        104 | (0,104) |  1
        105 | (0,105) |  1
          1 | (0,1)   |  1
(5 rows)

postgres=# explain select row_number() over(),ctid,* from t order by id desc
offset 100 limit 5;
                              QUERY PLAN
----------------------------------------------------------------------
 Limit  (cost=33.79..33.80 rows=5 width=10)
   ->  Sort  (cost=33.54..34.79 rows=500 width=10)
         Sort Key: id DESC
         ->  WindowAgg  (cost=0.00..14.25 rows=500 width=10)
               ->  Seq Scan on t  (cost=0.00..8.00 rows=500 width=10)
(5 rows)