Reinstall Mountain Lion after factory reset to Lion
From Personal Experience: I believe it's the same for everyone. It is the only item in the Mac App Store that doesn't show purchased even though it is purchased, even in UK, however I don't think they would make you pay for it again as it is linked to your account. You could call them and ask them or ask a Genius in store, I would just to be sure you don't get billed again for it, even though you shouldn't.