Are you referring to Microgaming download client only casinos?
These as I remember provide you with the message 'Your account is already registered' when downloading/re-installing the software.
A blessing if you ask me, protects you without having to remember!!
This is something stored locally in the Windows registry even after a Viper casino is uninstalled. It is not something players should rely on. It's easy to bypass, both deliberately and unknowingly.
The best way for players to protect themselves is to keep a list of all the accounts they have ever opened. They can refer to this list whenever a "new" casino takes their fancy. It can get tricky when casinos change their name and software as the only way for a player to track this on their list is to be aware of the industry in general, such as by being a member of Casinomeister.
IP address is a poor way of detecting multiple accounts, as IP addresses change, and for some people can change every time they connect to the internet. Email address isn't any better because people can create as many as they like, and sometimes email providers close down or change domain, which effectively produces a change of email address.
The Microgaming system, based on information held on a particular computer, only lasts for the life of that installation of Windows, and this can be just a couple of years (thanks Microsoft

). It is also easy to bypass by those deliberately trying to bypass the block on account creation.
Currently, there is no really good way of identifying possible multiple accounts, and so the player kept list is the best way to ensure not to inadvertently break the rules. Fraudsters can easily bypass these systems and create the accounts, but passing KYC is harder, and is getting harder all the time.
The industry could improve upon the KYC process for the benefit of innocent players by cooperating and creating some kind of industry wide KYC verification system so that one is either validated at all casinos, or fails KYC at all casinos. They would also need to recognise that sometimes they DO get it wrong, and need a proper appeals channel for wrongly failed KYC checks just as they already have a system to catch those who passed KYC, but should have failed.
In terms of Windows, Microsoft could help such internet security by issuing personal licence codes to users, rather than licence codes for specific machines. This would mean that each household at least could be identified through a unique licence code for all Windows installations on all machines owned by members of that household. A shift to IPv6 would also give each household it's own fixed IP address. These together would provide two security checks that would normally remain fixed over time, with any changes far less likely to be produced by spurious happenings in the internet system.
A better means for identifying the current physical location of a player already exists. It has been developed because the US state authorities in New Jersey refused to accept the current methods based on geolocation due to it's inaccuracy, and in particular the risk that someone physically outside the state might be geolocated as being inside and permitted to play.