SQL & PL/SQL

Hi Experts,

I'm doing a string aggregation but not able to get Distinct values aggregated.
Ex -

with xx as(
  select 'A' a from dual union all
  select 'B' a from dual union all
  select 'C' a from dual union all
  select 'C' a from dual union all
  select 'D' a from dual 
)
select rtrim(xmlagg(xmlelement(e,x.a||',')).extract('//text()'),',') from xx x;

gives

A,B,C,C,D

Expected o/p :

A,B,C,D

Is there any other String Aggregation techniques which can achieve this?
Please suggest ideas.

Thanks in Advance.
Ranit B.

Oracle Database version - 11.2.0.2.0