If you're having the *This card number has already been used* issue but destiny still hasn't started to download, follow these instructions.
1: Go to the PSN Store
2: Go to the games category
3: Hit X on New Games This Month (or something of the like)
4: On the list will be destiny, click on it, and you will be allowed to download it.
On the other hand, if you keep getting maintenance errors or errors when you try to "purchase" the beta, that's PSN telling you to go away because it's trying to process everything else, by trying over and over you're bogging down the system for everyone else, so wait a half hour and come back.
Hope this helped.