Oracle 18c

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create table tbl (col) as
    select 'A' from dual union all
    select 'C' from dual union all
    select 'A' from dual union all
    select 'B' from dual
/ 
4 rows affected

select json_object ('ArrayKey' value json_arrayagg (col order by col)) jsonResult
from (select col from tbl group by col)


JSONRESULT


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


SELECT /*+NO_MERGE(x)*/ json_object(
     'ArrayKey' VALUE json_arrayagg(col)) jsonResult
FROM (SELECT DISTINCT col FROM tbl) x;


JSONRESULT


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


WITH cte AS (
    SELECT /*+MATERIALIZE*/ DISTINCT col
    FROM tbl
)
SELECT json_object(
    'ArrayKey' VALUE json_arrayagg(col)) jsonResult
FROM cte;


JSONRESULT


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