create table relation(
 id int primary key auto_increment,
 userId int not null,
 fanId int not null
 );

insert into relation(userId,fanId) 
values(1,1) ,(1,1) ,(1,1), (2,2),(2,2) ,(3,3),(3,3);

delete t from relation s
join relation t using(userId,fanId)
where s.id<t.id;