Are they explicitly telling you to send another bitcoin, or they request you to open up a new mixing session with the same details to get a new wallet key (without sending another bitcoin)? I'm not familiar with how they work but their last message seems a bit unclear to me. Regardless of what it is, I think they should have a better refund procedure. If they really ask you to send new coins, I feel like they'll exit scam soon.
When a user creates a new sudoku wallet, he gets a zero balance account and access key, bitcoin address and private key from this address. The user can send any amount of coins for mixing.
When I was asked to create a new mixing for the same amount, this clearly implies sending new coins.