почему же? ошибаешься... можно например так:
select username from phpbb_users group by user_id limit 10 union all select username from phpbb_users;
в таком случае и с повторами выйдет.
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а можно и так:
select username from phpbb_users group by user_id limit 10 union select 'XAXA' from phpbb_users;
или так
select username from phpbb_users group by user_id limit 10 union all select 0x58415841 from phpbb_users;
или так
select username from phpbb_users group by user_id limit 10 union all select 1000001 from phpbb_users;