MySQL 8.0

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create table mytable (
  recnr	int,
  test_id int
);

insert into mytable values
(1,	101),
(2,	102),
(3,	102),
(4,	103),
(5,	104),
(6,	104),
(7,	104),
(8,	105);

Records: 8  Duplicates: 0  Warnings: 0


select test_id
from mytable
group by test_id
having count(1) > 1;


test_id


102

104


select t.*
from mytable t
inner join (
  select test_id
  from mytable
  group by test_id
  having count(1) > 1
) as s on s.test_id = t.test_id


recnr
test_id


2
102

3
102

5
104

6
104

7
104