-- 建表
create table a(
id int primary key auto_increment,
contents varchar(32)
);
--插入测试数据
insert into a values(default,'AA');
insert into a values(default,'BB');
insert into a values(default,'AA');
insert into a values(default,'CC');
insert into a values(default,'BB');
insert into a values(default,'DD');
insert into a values(default,'EE');
insert into a values(default,'FF');
insert into a values(default,'FF');
-- 查出所有重复数据
select id,contents from (
    select * from (select  * from a) aa
        inner join
            (select id idd,contents cons from a) bb
                where aa.id!=bb.idd and aa.contents = bb.cons
) rst order by id asc;

-- 查处重复数据中的一条 如：AA BB FF重复了 只查处一个AA 一个BB 一个FF
select distinct(contents)  from (
select   id,contents  from (
    select * from (select  * from a) aa
        inner join
            (select id idd,contents cons from a) bb
                where aa.id!=bb.idd and aa.contents = bb.cons
) rst order by id asc
) t;

 其他方法1：

select aa.id,aa.contents from a as aa,(select contents from a group by contents HAVING count(*)>1) bb
where aa.contents = bb.contents 

 其他方法2：
select contents from a as aa where (select count(contents) from a as b where aa.contents = b.contents)>1