Postgres array union of result set

test.sql

CREATE TABLE test (
  some_arrays int[]
);

INSERT INTO test (some_arrays) VALUES (array[1,2,3]), (array[3,4,5]), (array[5,6,7]), (array[7,8,9]);

WITH first_query AS (
  SELECT 1 AS arbitrary_group_by, unnest(some_arrays) AS my_numbers 
  FROM test
)
SELECT array_agg(distinct my_numbers) 
FROM first_query 
GROUP BY arbitrary_group_by;


-> Returns {1,2,3,4,5,6,7,8,9}