select
  r.first_name    ,
  r.last_name     ,
  u.email

from (
  select distinct
    lower(first_name) first_name,
    lower(last_name)  last_name

  from
    users
) r, users u

where
  u.first_name = r.first_name and
  u.last_name = r.last_name

order by
  r.first_name,
  r.last_name
;