One user registered four times, and all four are legitimate

This Habr article discusses a common issue with user account duplication caused by improper email address handling. The author notes that many systems compare email addresses character-by-character without considering provider-specific rules. For instance, Gmail ignores dots in usernames and characters following a '+' sign, and is case-insensitive. Consequently, a single user can legitimately create multiple accounts using variations of the same address. This creates vulnerabilities in business logic, allowing users to abuse new-user promotions, bypass free-tier limits, or violate 'one entry per person' policies. To address this, the author recommends normalizing email addresses before storing them in the database, converting them to a standard format to prevent duplication and unauthorized resource consumption.