Postgres 11

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CREATE TABLE lines (
  client integer NOT NULL,
  block text NOT NULL,
  date date NOT NULL
);

INSERT INTO lines (client, block, date) VALUES
  (1, 'P10001', '2020-01-01'),
  (1, 'P10002', '2020-04-17'),
  (1, 'P10002', '2020-05-04'),
  (1, 'P10003', '2020-05-05');
4 rows affected

select l.* from lines l
where exists (
  select 1 from lines
  where client = l.client and block = l.block and date <> l.date
)


client
block
date


1
P10002
2020-04-17

1
P10002
2020-05-04


select client, block, date
from (
  select *, count(*) over (partition by client, block) counter
  from lines
) t
where counter > 1


client
block
date


1
P10002
2020-04-17

1
P10002
2020-05-04