LISTAGG en Oracle para devolver valores distintos

19c y posteriores:

select listagg(distinct the_column, ',') within group (order by the_column)
from the_table

18c y anteriores:

select listagg(the_column, ',') within group (order by the_column)
from (
   select distinct the_column 
   from the_table
) t

Si necesita más columnas, algo como esto podría ser lo que está buscando:

select col1, listagg(col2, ',') within group (order by col2)
from (
  select col1, 
         col2,
         row_number() over (partition by col1, col2 order by col1) as rn
  from foo
  order by col1,col2
)
where rn = 1
group by col1;