sorting/grouping/(non-)unique indexes bug

Greetings!

Before I present the following, I must say that 'simple' index
corruption is highly improbable; the server did not suffer hard
reboots.

rdw=# \d reg_user
Table "reg_user"
Column | Type | Modifiers
------------------+------------------------+------------------------------------------------
user_id | integer | not null default nextval('reg_user_seq'::text)
user_email | character varying(50) | not null
user_passwd | character varying(32) | not null
user_active | boolean | not null default 't'
user_allow_pauth | boolean | default 'f'
user_full_name | character varying(100) |
user_pseudonym | character varying(100) |
user_who | character(1) |
Primary key: reg_user_pkey
Unique keys: reg_user_email_key

rdw=# \d reg_user_email_key
Index "reg_user_email_key"
Column | Type
------------+-----------------------
user_email | character varying(50)
unique btree

rdw=# create table broken as select user_email from reg_user;
SELECT

rdw=# -- one may think that emails are unique...
rdw=# select count(user_email) from broken;
count
--------
212400
(1 row)

rdw=# select count(distinct user_email) from broken;
count
--------
212397
(1 row)

rdw=# -- look closely at the numbers
rdw=# select user_email, count(user_email) from broken group by user_email order by 2 desc;
user_email | count
----------------------------------------------------+-------
[duplicated email] | 2
[some email] | 1
[some email] | 1
[some email] | 1

rdw=# delete from broken where user_email = '[duplicated email]';
DELETE 2

rdw=# select user_email, count(user_email) from broken group by user_email order by 2 desc;
user_email | count
----------------------------------------------------+-------
[another email] | 2
[some email] | 1
[some email] | 1

rdw=# delete from broken where user_email = '[another email]';
DELETE 2

rdw=# select user_email, count(user_email) from broken group by user_email order by 2 desc;
user_email | count
----------------------------------------------------+-------
[some email] | 1
[some email] | 1
[some email] | 1

rdw=# --finally, no more duplicates!
rdw=# create unique index broken_email_key on broken (user_email);
ERROR: Cannot create unique index. Table contains non-unique values

rdw=# select version();
version
-------------------------------------------------------------
PostgreSQL 7.2.1 on i686-pc-linux-gnu, compiled by GCC 2.96
(1 row)

$psql -l
List of databases
Name | Owner | Encoding
-----------+----------+-----------
...
rdw | alex | WIN
...

The server is run under ru_RU.CP1251 locale, initdb was done under the
same locale.

As you probably guessed, reg_user table contains actual email
addresses of our site's registered users. I can provide them for
testing (without passwords and stuff, of course) if this is going to
help.

--
Yours, Alexey V. Borzov, Webmaster of RDW.ru