An sql example to remove duplicate fields in a table from by one column.

sql_remove_duplicates.sql

drop table testtb;
drop table test_duplicated;
drop table test_unique;

create temporary table testtb (
    entity varchar(10) not null,
    duplicate varchar(10) not null
);

insert into testtb
values
('a', 'b'),
('b', 'a'),
('a', 'c'),
('c', 'b'),
('c', 'x'),
('b', 'x');

-- Traigo cantidad duplicados
select entity,count(*)
from testtb
group by entity
having count(*)>1;

-- Cargo todas las que tienen duplicados
SELECT a.*
into test_duplicated
FROM testtb a
JOIN (
	SELECT entity, COUNT(*)
	FROM testtb 
	GROUP BY entity
	HAVING count(*) > 1 
) b
ON a.entity = b.entity
ORDER BY a.entity

--Filtro por la 1ra
select distinct ttb.entity,ttb.duplicate
into test_uniques
from testtb as ttb
inner join test_duplicated as td on td.entity=ttb.entity
where ttb.duplicate=(
	select a.duplicate
	from test_duplicated as a
	where a.entity=ttb.entity
	limit 1
)

--Aplico borrado
delete from testtb as tb
where not exists (select * from test_uniques as x where x.entity=tb.entity and x.duplicate=tb.duplicate)

select * from testtb