SQL duplicates with different primary keys

Question

I have a table in this form:

id | firstname | lastname
---+-----------+----------
1  | alex      | marti
2  | mark      | finger
3  | alex      | marti
4  | ted          


        

Answer 1

select id, firstname, lastname
from table t
where exists (select 1
from table t2
where t2.firstname = t.firstname
and t2.lastname = t.lastname
and t2.id <> t.id)