Postgres 13

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CREATE TABLE my_table (
    the_visitor_id varchar(5) NOT NULL, 
    the_visitor_visit timestamp NOT NULL,
    the_visitor_returning text
);

INSERT INTO my_table
VALUES ('VIS01', '2019-05-02 09:00:00','YES' ), 
       ('VIS01', '2019-05-04 12:00:00',NULL ), 
       ('VIS01', '2019-05-05 18:00:00',NULL ), 
       ('VIS02', '2019-05-06 18:30:00',NULL), 
       ('VIS02', '2019-05-15 12:00:00',NULL), 
       ('VIS03', '2019-06-30 18:00:00','YES'),
       ('VIS04', '2019-06-30 18:00:00','NULL');
7 rows affected

SELECT the_visitor_id, the_visitor_visit, the_visitor_returning 
FROM   my_table t1
WHERE  EXISTS (
   SELECT FROM my_table
   WHERE  the_visitor_id = t1.the_visitor_id
   AND    ctid <> t1.ctid
   );


the_visitor_id
the_visitor_visit
the_visitor_returning


VIS01
2019-05-02 09:00:00
YES

VIS01
2019-05-04 12:00:00
null

VIS01
2019-05-05 18:00:00
null

VIS02
2019-05-06 18:30:00
null

VIS02
2019-05-15 12:00:00
null


SELECT the_visitor_id, the_visitor_visit, the_visitor_returning 
FROM  (
    SELECT *, count(*) OVER (PARTITION BY the_visitor_id) AS ct
    FROM   my_table
    ) sub
WHERE  ct > 1;


the_visitor_id
the_visitor_visit
the_visitor_returning


VIS01
2019-05-02 09:00:00
YES

VIS01
2019-05-04 12:00:00
null

VIS01
2019-05-05 18:00:00
null

VIS02
2019-05-06 18:30:00
null

VIS02
2019-05-15 12:00:00
null

the_visitor_id	the_visitor_visit	the_visitor_returning
VIS01	2019-05-02 09:00:00	YES
VIS01	2019-05-04 12:00:00	null
VIS01	2019-05-05 18:00:00	null
VIS02	2019-05-06 18:30:00	null
VIS02	2019-05-15 12:00:00	null