The next block is 'return to ravinika,' so that's why. Some of the old ravinika cards will probably be reprinted in the new set.
There's no list of what's actually in the duel decks set, apart from the two cards you can see on the front. But since buying an old Niv-mizzet doesn't cost that much less than buying the set (!), it's worth it.
two 60-card decks, including a reprinted niv-mizzet card with a new picture = £15
one Niv-mizzet card from the old ravinika set = £12!
Perhaps next time we meet up we can do some sealed games for real or something?
Hmm, maybe! Bit expensive though. Booster packs are like 3.50 each.