pg_dump ERROR, usename "postgres" duplicated

Hi:
    I cannot dump any database from my server. When I try to do it i receive this error

pg_dump: The command was: SELECT tableoid, oid, nspname, (select usename from pg_user where nspowner = usesysid) as usename, nspacl FROM pg_namespace

    I checked the pg_user database and I found this:

postgres=# select * from pg_user;
    usename    | usesysid | usecreatedb | usesuper | usecatupd |  passwd  | valuntil | useconfig
---------------+----------+-------------+----------+-----------+----------+----------+-----------
 postgres      |        1 | t           | t        | t         | ******** |          |
 postgres      |        1 | t           | t        | t         | ******** |          |
 administrador |      100 | t           | t        | t         | ******** |          |
 user1         |      101 | t           | t        | t         | ******** |          |
 user2         |      102 | f           | f        | f         | ******** |          |

    Anyone can tell me from where should I start?. This is a production server.


Regards.

--
Gastón Quiroga