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