It should be guaranteed (I checked the code), BUT you need to make sure the two legendaries are not identical (due to a bug, not intentional), which, if we're thinking about the same thing, happened to that person.
(well, that's what I think happened, anyway. Haven't heard official confirmation but had similar issues when researching other things)
Should bugs happen and it's single player, you can always "cheat" the artefact into your inventory, too.