Julien Schröter authored Jun 27, 2020 and Julien Schröter committed Sep 14, 2020

Email address and username must always be unique. Register will answer
with HTTP status code 409 Conflict when a user tries to use an existing
username or tries to reuse an email address to create a new account.

The endpoint will also return an json object with a field "field"
providing information which attribute failed.

When the username and the email address are both reused, the field will
have the value "username".