Before starting gambling there, you must have read their Terms and Conditions where they have clearly mentioned that they may require KYC anytime they want. Therefore, I see nothing wrong with that as they have coded the terms. You agreed with their terms and condition while registering, didn't you? Why without a read?
You have been accused of multi-accounting, isn't it fair enough to ask KYC?
4.2. We reserve the right to verify Your registration details and all other personal data provided by You to Us any time after the registration of the Account, such as : name, surname, identity information, personal number and passport number, address, age and any other personal data, at any time, by requesting certain type of documents.
https://fortunejack.com/faq/terms_and_conditions