I don't know what would happen if the same image went up twice, but I often see the identical image a few times on Alamy, maybe because of this sort of scenario. They say it hits your CTR, therefore your Alamy Rank.
I'm guessing that Zoonar would cream off some of the Alamy payment for themselves? Therefore unless you make loads of money from the Zoonar site, it would not be worth it. Can you opt out of the Alamy partnership from Zoonar and upload to Alamy directly? If you don't have time to double submit, that would also be a consideration.
Check out what Zoonar creams off, then decide if the convenience is worth it for you.