he can also create a new user

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There is a very robust way to prevent or slowdown multiple registrations: ask every sign up applicant to submit an essay on why he wants to create an account. The essay should be two A4 pages of clean and solid argumentation. Similarly to one you write when applying for some job position.

That is more robust than a phone number confirmation. IIRC, habr.com used such system to prevent spam. In general, most Kremlin bots would be out.

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