Oracle 18c

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CREATE TABLE tbl_name
AS
SELECT 'A' column_name
       FROM dual
UNION ALL
SELECT 'C' column_name
       FROM dual
UNION ALL
SELECT 'A' column_name
       FROM dual
UNION ALL
SELECT 'B' column_name
       FROM dual;
4 rows affected

SELECT json_object('ArrayKey'
                   VALUE json_arrayagg(column_name)) jsonresult
      FROM (SELECT DISTINCT
                   column_name
                   FROM tbl_name) x;


JSONRESULT


{"ArrayKey":["A","C","A","B"]}


WITH cte
AS
(
SELECT DISTINCT
       column_name
       FROM tbl_name
)
SELECT json_object('ArrayKey'
                   VALUE json_arrayagg(column_name)) jsonresult
       FROM cte;


JSONRESULT


{"ArrayKey":["A","C","A","B"]}


SELECT /*+NO_MERGE(x)*/
      json_object('ArrayKey'
                  VALUE json_arrayagg(column_name)) jsonresult
      FROM (SELECT DISTINCT
                   column_name
                   FROM tbl_name) x;


JSONRESULT


{"ArrayKey":["B","C","A"]}


WITH cte
AS
(
SELECT /*+MATERIALIZE*/ 
       DISTINCT
       column_name
       FROM tbl_name
)
SELECT json_object('ArrayKey'
                   VALUE json_arrayagg(column_name)) jsonresult
       FROM cte;


JSONRESULT


{"ArrayKey":["A","C","B"]}