SQL select with unique_list - losing an item
There are two ways: first - using regexp_replace like here - Eliminate duplicates in ListAgg (Oracle), second - use distinct inside a subquery:

select fk_proddes_id,fk_id_cod_deseu, 
       listagg(to_char(cod_eliminare), ',') within group (order by cod_eliminare desc)
  from (select distinct fk_proddes_id,fk_id_cod_deseu, cod_eliminare
          from sim_sd.proddes_ag_elim)
 group by fk_proddes_id, fk_id_cod_deseu