How can Heroku ensure passwords are not duplicated across accounts?

I got the following email from Heroku:

> As part of our efforts to enhance our security and in response to an incident published on [status.heroku.com](https://status.heroku.com/incidents/2413), we wanted to inform you that we will begin resetting user account passwords on May 4, 2022. We recommend that you reset your user account password in advance here and follow the best practices below:
>
> - Minimum of 16 characters
> - Minimum complexity of 3 out of 4: Uppercase, Lowercase, Numeric, …

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