DO $$
DECLARE

_a1 jsonb;
_a2 jsonb;
_a3 jsonb;

BEGIN

_a1 := (select '["A","B","A"]'::jsonb);
_a2 := (select '["A","B","A"]'::jsonb);
SELECT 
    jsonb_agg(DISTINCT x.ae)
INTO
    _a3
FROM
    (
    SELECT jsonb_array_elements(_a1) ae
    UNION ALL 
    SELECT jsonb_array_elements(_a2) ae
) x;

raise notice '%', _a3;

END;
$$
language plpgsql