When it comes to multi-accounting then this would vary on each site because basing off on experience with most one game dedicated site like dice where every users had the chance or freely create accounts as much as they want as for others do limit out only single account for each ip.
Online gambling sites have stringent rules on multi-accounting as they run many promotions or giveaways, contests on their gambling sites. Multi-accounts will compromise their events and it is terrible to allow multi-accounts to cheat their companies with free money. If they win big, they will make big withdrawal request and if accepted, it will cause big losses for gambling sites.
Just like on what happened on luckybit where i do make some new account and when tending to make out some cashouts, it was put on hold due to that multi-accounting reason.
Detecting multiple accounts in one IP shouldnt really be a problem.
Detecting multi-accounts is not difficult for gambling sites but to protect their services and reputation, rules in ToS, FAQ need to be clear. If they arbitrarily reject withdrawal, suspend accounts, their reputation will be damaged.