pg_dumpall problem - duplicated users

Hi,

few days ago we made vacuum of all databases on our server. After that
we canot dump databases:

# pg_dumpall -i -U postgres > test.sql
pg_dump: query to obtain list of schemas failed: ERROR: More than one
tuple returned by a subselect used as an expression.
pg_dumpall: pg_dump failed on database "alibi", exiting

There are duplicated system users, pg_catalogs, pg_temp1, pg_toast and
public...

template1=> select * from pg_catalog.pg_shadow where usesysid = 1;
usename | usesysid | usecreatedb | usesuper | usecatupd
| passwd | valuntil | useconfig
----------+----------+-------------+----------+-----------+-------------------------------------+----------+-----------
postgres | 1 | t | t | t
| | |
postgres | 1 | t | t | t |
md5c084502ed11efa9d3d96d29717a5e555 | |
(2 rows)

Any suggestions how to solve the problem? There are many databases on
the server we can't loose. Applications can connect to databases - all
works fine, hoewer we must make dumps...

Thanks for any help,
Greetings

Przemek