Oh ok, found it:
# select one.username, one.user_id, two.user_id from phpbb_users one,
phpbb_users two where two.username=one.username and two.user_id <>
one.user_id;
username | user_id | user_id
----------+---------+---------
Вячеслав | 7564 | 421
Вячеслав | 421 | 7564
(2 rows)
Sorry!