how to look for duplicate rows?

i have to clean a table that looks like so:

create table test (sn integer, fname varchar(10), lname varchar(10));
insert into test values (1, 'adam', 'lambert');
insert into test values (2, 'john', 'mayer');
insert into test values (3, 'john', 'mayer');
insert into test values (4, 'mary', 'kay');
insert into test values (5, 'john', 'mayer');
insert into test values (6, 'susan', 'boyle');
insert into test values (7, 'susan', 'boyle');
insert into test values (8, 'mark', 'ingram');

for that, i need to run a query that returns like so:

result:
is_not_distinct
2, 3, 5
6, 7

using 8.1.