SELECT WM_CONCAT(DISTINCT '"'||ename||'"')  E
      FROM emp
E
-------
"ADAMS","ALLEN","BLAKE","CLARK","FORD","JACK","JAMES","JONES","KING","MACK","MARTIN","MIKE","MILLER","SCOTT","SMITH","TURNER","WARD"

SQL> CREATE OR REPLACE PROCEDURE test23 IS
  2    coll VARCHAR2(100);
  3  BEGIN
  4        SELECT WM_CONCAT(DISTINCT '"'||ename||'"')  E
  5        INTO coll
  6        FROM emp;
  7  END;
  8  /

Warning: Procedure created with compilation errors.

SQL> show err
Errors for PROCEDURE TEST23:

LINE/COL ERROR                                                                  
-------- -----------------------------------------------------------------      
4/7      PL/SQL: SQL Statement ignored                                          
4/14     PL/SQL: ORA-30482: DISTINCT option not allowed for this function 

select wm_concat('"'||ename||'"')
from (select distinct ename from emp)
/

SQL> CREATE OR REPLACE PROCEDURE test23 IS
  2    coll VARCHAR2(100);
  3  BEGIN
  4    select wm_concat('"'||ename||'"')
  5    INTO coll
  6    from (select distinct ename from emp)
  7    ;
  8  END;
  9  /

Procedure created.