Double error message when trying to register account with blacklisted username

### Steps to reproduce
1. To to `/register`
2. Enter `twitter` as the username for the new account, and enter a valid email and password
3. Submit the form
### Expected behaviour

A single error message. "An account with this username already exists" is not really correct but will do.
### Actual behaviour

"An account with this username already exists" error message is shown twice.


This does not seem to happen when trying to register an account for a non-blacklisted username where an account with the username actually does already exist - you'll get the error message only once.

Start at the `/register` entry point and reproduce the blacklisted-username case with `twitter`, then compare it with registration using an ordinary existing username. Trace the registration validation and error-display flow to identify why the same message appears twice. Done means the blacklisted-username case displays one error message, without changing the stated behavior for ordinary duplicate usernames.

Written by the indexing model from the issue text.