ERROR: cannot insert duplicate... on VACUUM ANALYZE

Hello-

When running a routine VACUUM ANALYZE on one of our databases (pg
v7.1.2) the following message appears:

main_v0_8=# VACUUM ANALYZE ;
ERROR:  Cannot insert a duplicate key into unique index admin_users_pkey

This primary key is referring to the following table:

main_v0_8=# \d admin_users
                                       Table "admin_users"
   Attribute   |           Type           |
Modifier
--------------+--------------------------+------------------------------------------------------
  id           | smallint                 | not null default
nextval('admin_users_id_seq'::text)
  name         | character varying(255)   | not null
  password     | character varying(20)    | not null
  email        | character varying(255)   |
  fullname     | character varying(255)   |
  usertype     | smallint                 | not null default 1
  mygroup      | smallint                 | not null default 1
  active       | boolean                  | not null default 't'
  remoteip     | character varying(15)    |
  sessioncode  | character varying(20)    |
  lastconnect  | timestamp with time zone |
  lastip       | character varying(15)    |
  firstconnect | timestamp with time zone |
Indices: admin_users_name_key,
          admin_users_pkey

Any suggestions on how to track down what is happening?  I am not
actually trying to insert anything to this table, so this error message
appears a bit strange to me.

One other bit of info, one (of the approx 40) users of this database is
receiving the same error message and is unable to work, while the other
users are having no problems.  Strange.

--

-**-*-*---*-*---*-*---*-----*-*-----*---*-*---*-----*-----*-*-----*---
  Jon Lapham
  Extracta Moléculas Naturais, Rio de Janeiro, Brasil
  email: lapham@extracta.com.br      web: http://www.extracta.com.br/
***-*--*----*-------*------------*--------------------*---------------