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 DISTINCT col as col FROM tbl
/

COL

B

C

A

SELECT JSON_OBJECT(
  'ArrayKey' VALUE JSON_ARRAYAGG(col) 
) AS jsonResult
FROM (SELECT DISTINCT col as col FROM tbl);

JSONRESULT

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

SELECT JSON_OBJECT (
  'ArrayKey' VALUE JSON_ARRAYAGG(DISTINCT col) 
) AS jsonResult
FROM (SELECT DISTINCT col AS col FROM tbl);

JSONRESULT

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