According to this post at Stack Overflow, MariaDB 10.1.10 gives wrong results (duplicate rows) for UNION DISTINCT while the UNION version gives correct ones.

(notice the comment: http://dba.stackexchange.com/questions/141799/select-distinct-values-of-two-columns-as-single#comment268612_141800 )

SELECT a AS result FROM tableX
UNION DISTINCT
SELECT b FROM tableX ;