proc sql;
create table want as
select * from a group by x having count(x)>1;
quit;

data b;
        input x $ y @@;
datalines;
A 1 A 2 B 3 C 4 C 5 C 6
;
RUN;
proc sql;
        create table b1 as
         select *, sum(y) as sum
         from b
         group by x;
quit;
data b2;
        set b1;
        if sum^=y;
        drop sum;
run;

新手，不怎么简单，但是也提供了一种思路哈：
proc sort data=B nodupkey out=Bdistinct dupout=bdup;
        by x;
run;

proc sort data=bdup;
        by x;
run;
proc sort data=B;
        by x;
run;
data final;
        merge bdup(in=a) B;
        by x;
        if a;
run;

学习了

学习了