I don't think the Betfury team checks each user manually. Betfury probably has a special algorithm that analyzes the behavior on the site, cookies, and user IP addresses. It seems to me this should be enough to successfully detect abusers using multiple accounts. In addition, the rules of the Funfury competition indicate that if Betfury detects multi-accounting abuse, they can seize the winnings from the abuser's balance.
You're right, I believe every casino has a special algorithm to prevent multi-accounts and filter every IP or cookie that is the same to make it easier to identify suspicious accounts, if abuse is found it will violate the TOS about multi accounts so that the account can be suspended.