How to use Oracle's LISTAGG function with a unique filter? [duplicate]

I have a table like this:

group_id  name  
--------  ----
1         David
1         John
1         Alan
1         David
2         Julie
2         Charles

And I want the following result:

group_id  names
--------  -----
1         'Alan, David, John'
2         'Charles, Julie'

I can use the following query:

select group_id, 
       listagg(name, ',') within group (order by name) as names
from demotable
group by group_id 

To get this (very similar result):

group_id  names
--------  -----
1         'Alan, David, David, John'
2         'Charles, Julie'

Any ideas how I can filter the names by uniqueness in the LISTAGG call?

I don't have an 11g instance available today but could you not use:

SELECT group_id,
       LISTAGG(name, ',') WITHIN GROUP (ORDER BY name) AS names
  FROM (
       SELECT UNIQUE
              group_id,
              name
         FROM demotable
       )
 GROUP BY group_id

Super simple answer - solved!

my full answer here it is now built in in some oracle versions.

select group_id, 
regexp_replace(
    listagg(name, ',') within group (order by name)
    ,'([^,]+)(,\1)*(,|$)', '\1\3')
from demotable
group by group_id;  

This only works if you specify the delimiter to ',' not ', ' ie works only for no spaces after the comma. If you want spaces after the comma - here is a example how.

select 
replace(
    regexp_replace(
     regexp_replace('BBall, BBall, BBall, Football, Ice Hockey ',',\s*',',')            
    ,'([^,]+)(,\1)*(,|$)', '\1\3')
,',',', ') 
from dual

gives BBall, Football, Ice Hockey

create table demotable(group_id number, name varchar2(100));
insert into demotable values(1,'David');
insert into demotable values(1,'John');
insert into demotable values(1,'Alan');
insert into demotable values(1,'David');
insert into demotable values(2,'Julie');
insert into demotable values(2,'Charles');
commit;

select group_id, 
       (select listagg(column_value, ',') within group (order by column_value) from table(coll_names)) as names
from (
  select group_id, collect(distinct name) as coll_names 
    from demotable
    group by group_id 
)

GROUP_ID    NAMES
1   Alan,David,John
2   Charles,Julie