Aggregate and concatenate strings without duplication - sql

Question

I want to aggregate strings and concatenate them. This is example of what I am using
SELECT deptno, LISTAGG(ename, ',') WITHIN GROUP (ORDER BY ename) AS employees
FROM emp
GROUP BY deptno;
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK, CLARK,KING,MILLER, MILLER
20 ADAMS, ADAMS, ADAMS, FORD,JONES,SCOTT,SMITH
30 ALLEN,BLAKE,JAMES,MARTIN,MARTIN,TURNER,WARD
3 rows selected.
But I want results without duplication in concatenated strings.
Desired results:
DEPTNO EMPLOYEES
---------- --------------------------------------------------
10 CLARK,KING,MILLER
20 SMITH,FORD,ADAMS,SCOTT,JONES
30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD
3 rows selected.
EDIT: so in example below there are some "Employeer names with comma". It is using the proposed answer with regular expression:
with emp as (
select 10 as deptno, 'clark,1' as ename from dual
union all
select 10, 'clark,1' from dual
union all
select 10, 'clark,1' from dual
union all
select 10, 'bob' from dual
union all
select 10, 'bob' from dual
union all
select 10, 'don' from dual
union all
select 20, 'tim,2' from dual
union all
select 20, 'tim,2' from dual
union all
select 20, 'tim,2' from dual
union all
select 20, 'jack' from dual
union all
select 20, 'mark' from dual
)
SELECT e.deptno, --LISTAGG(e.ename, ',') WITHIN GROUP (ORDER BY e.ename) AS employees,
RTRIM(REGEXP_REPLACE(listagg (e.ename, ',')
WITHIN GROUP (ORDER BY e.ename),
'([^,]+)(,\1)+', '\1'),
',') AS employees
FROM emp e
GROUP BY e.deptno;
The results of this query are not correct:

Answer 1

Replace
FROM emp
with
FROM (SELECT DISTINCT deptno, ename FROM emp)
(From a non-Oracle user, but it should work)

Answer 2

You can use REGEX:
SELECT deptno,
rtrim( regexp_replace( (Listagg(ename,',') within GROUP (ORDER BY ename) OVER ()), '([^-]*)(-\1)+($|-)', '\1\3'), '-') within GROUP (ORDER BY ename) AS employees
FROM emp
GROUP BY deptno;
Here's a method using crossjoin
SELECT DISTINCT deptno,
b.enames
FROM emp a
cross join (SELECT Listagg(ename, '-')
within GROUP (ORDER BY ename) enames
FROM (SELECT DISTINCT enam ename
FROM emp)) b
Let me know if they work