BUG #10429: the same update return different result

The following bug has been logged on the website:

Bug reference:      10429
Logged by:          zhanglong
Email address:      chuisiniu@hotmail.com
PostgreSQL version: 9.3.4
Operating system:   window7 32bit
Description:

E:\postgresql-9.4beta1\db\bin>psql postgres
psql (9.4beta1)
Type "help" for help.

postgres=# drop table t1;
DROP TABLE
postgres=# create table t1(c1 int primary key);
CREATE TABLE
postgres=# insert into t1 values (1);insert into t1 values (2);update t1 set
c1 = c1 +1;
INSERT 0 1
INSERT 0 1
ERROR:  duplicate key value violates unique constraint "t1_pkey"
DETAIL:  Key (c1)=(2) already exists.
postgres=# select * from t1;
 c1
----
  1
  2
(2 rows)


postgres=#
postgres=# drop table t1;
DROP TABLE
postgres=# create table t1(c1 int primary key);
CREATE TABLE
postgres=# insert into t1 values (2);insert into t1 values (1);update t1 set
c1 = c1 +1;
INSERT 0 1
INSERT 0 1
UPDATE 2
postgres=# select * from t1;
 c1
----
  3
  2
(2 rows)

The data is all the same except the order I insert it,but the update get
different result.From the source code, it seems that db update the tuple
first found out.