### One-sentence description ### Current behaviour Two registrations with the same email address ### Expected behaviour Only one registration/user per email address ### Steps to reproduce 1. Check https://thalia.nu/admin/registrations/registration/dd814d82-1a53-40a7-9b1c-19ee9b82ed18/change/ 2. Check https://thalia.nu/admin/registrations/registration/ed857356-e362-4493-bb0d-f546c9ffcacb/change/