Investigate double registrations

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/