On the plus side, a second purchase of ARR comes with another month of subscription time, so the extra cost isn't entirely wasted, as it saves you a month's sub. (The Heavensward expansion doesn't have that bonus, though, as it also would need to be purchased for each platform and it doesn't come with any subscription time.)

Though SE has said you need Heavensward on each platform you play it from, it's still not entirely clear how that works if you have the base game on multiple platforms and the expansion on only one of them.