How do you deal with multiple accounts per person?

Question

User identification

In addition to phone, email, and cookie verification options.
The algorithm itself is interesting. Well, the technical details will also come in handy.

Answer 1

It makes no sense to fight this - everything can be faked. And everything will be faked if there is a profit in it.
The mechanics of the service should be done in such a way that the clones do not bring benefits or bring it to the owners of the service. In general, a clone can be perceived as a full-fledged user.
For example, you can make paid features for which it makes sense to create clones. Most clone growers will count this out by themselves, and the most persistent ones will pay money.

Answer 2

https://github.com/Valve/fingerprintjs2

Answer 3

When registering, scan the retina, take DNA samples to resolve controversial issues.

Answer 4

SMS confirmation. If something is highly responsible, then only confirmation through physical presence and documents. Well, you can also have an EDS, if you work with legal entities.

Answer 5

Require during registration to replenish the internal balance by depositing real money. The internal balance can be spent on something useful inside the service, or returned in the form of real money when certain conditions are met.

Answer 6

You just let me invent bicycles. After all, everything has long been invented, written and tested, you just have to look around. If you have a purely academic interest - come up with it yourself, that's what interest is for. If you have a task - see how it is implemented by others. Most (Google, VK) use SMS confirmation. This is quite effective if you need to weed out 90% of twinks. If 100% unique accounts are important to you, look at the government services that are registered by SNILS in their offices.
Also, some services (for example, payment systems) combine these two methods. To register an account, a confirmation SMS is enough, but to get additional features (for example, accepting payments), you need to go through a more complex identity verification procedure.
PS
Do not forget that the more confused the registration, the fewer users. Don't overdo it.