WITH _cte_functions_rules AS (
    SELECT * FROM (
        SELECT x, y, count(1) AS xy_count FROM Functions GROUP BY x, y
    ) a WHERE x <> y OR xy_count > 1
)
SELECT least(f1.x, f1.y) AS x, greatest(f1.y, f1.x) AS f
  FROM _cte_functions_rules f1
  JOIN _cte_functions_rules f2
    ON f2.y = f1.x
   AND f2.x = f1.y
 GROUP BY 1,2
 ORDER BY x