Is Antike Duellum basically a two-player version of Antike II?
Yes, though it came first.
Is it worth getting if you already have Antike II?
Yes, definitely. Antike Duellum is a fantastic 2p experience. I think Antike plays best with 5 or 6, I've never even tried it with less than 4. Seems like a bad idea.
Polis is a much more complex and longer game, where Duellum has very simple rules. AFAoS is not so satisfactory as a dudes-on-a-map game imo.