I have 14 PCs at home. I can create as many logins as I want (almost) on every PC. That creates a high number of cookie sets. Furthermore, I can have as many (almost) e-mail accounts as I want.

"What prevents users from signing up multiple times...

Nothing as long as the user is ready to do it manually. And some users may be so committed.

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Everything went worng, just as foreseen.