with t(id1, id2) as (select 1, 1
from dual union all select 2, 1
from dual union all select 3, 1
from dual union all select 4, 7
from dual union all select 6, 7
from dual)
select id1#, id2
  from ( select t.*
              , nvl(max(decode(id2,id1,id2)) over(partition by id2), id1) id1#
           from t
) where id1=id1#;

      ID1#        ID2
---------- ----------
         1          1
         4          7
         6          7

select id1#, id2
from t
match_recognize(
partition by id2
order by id1
measures coalesce(s.id1, m.id1) id1#
one row per match
pattern(m* s m*|m)
define s as id1=id2
     , m as id1<>id2
)
;
      ID1#        ID2
---------- ----------
         1          1
         4          7
         6          7