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Some ISP's will share IP's between multiple users, which can make it difficult to distinguish them for the casinos, but they will generally only flag this behavior if there are lots of other red flags surrounding an account. Casinos know the common patterns, so if they get 20 people signing up from the same IP within a two day window, it's going to draw their attention - then they can view things like browser in use, screen dimensions, cookies stored and other fingerprinting which can flag someone who is not unique. This will trigger an investigation and all related accounts may end up having to prove they are different people.
Now not only will the IP be detected, the IP may be easily manipulated, but the device ID used will also be detected so that every user who uses a Multi-account will be caught.
There are many tricks to trick the system into escaping Multi-account, but the casino certainly has an antidote,
especially now that it is easier with the help of Ai and the like to auto-detect the Multi-account user.
But it's not impossible that they use fake KYC if the KYC implementation doesn't use face detectors,
But yes, there will always be a way to manipulate a system, so it will be determined who is one step ahead.