Oracle 11g Release 2

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with tbl(Code, Dt) as (
  SELECT 'c1', to_date('2020-10-01','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c1', to_date('2020-10-05','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c1', to_date('2020-10-09','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c1', to_date('2020-10-10','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c1', to_date('2020-10-20','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c2', to_date('2020-10-07','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c2', to_date('2020-10-09','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c2', to_date('2020-10-15','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c2', to_date('2020-10-16','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c2', to_date('2020-10-20','yyyy-MM-dd') FROM DUAL UNION ALL
  SELECT 'c2', to_date('2020-10-24','yyyy-MM-dd') FROM DUAL
),
pairs(Code, Dt, dtnext) as (
  select t1.Code, t1.dt, Min(t2.dt)
  from tbl t1
  join tbl t2 on t1.code=t2.code and t2.dt >= t1.dt + INTERVAL '7' DAY
  group by t1.Code, t1.dt
),
h(Code, Dtn) as (
  select Code,  Min(dt)
  from tbl
  group by Code
  union all
  select h.Code, p.dtnext
  from h
  join pairs p on p.code=h.code and p.Dt= h.dtn
  )
select * 
from h
order by code, dtn

CODE	DTN
c1	01-OCT-20
c1	09-OCT-20
c1	20-OCT-20
c2	07-OCT-20
c2	15-OCT-20
c2	24-OCT-20