Double submission of username in sign up

[peterbe]

https://sentry.prod.mozaws.net/operations/mdn-prod/issues/7710725/

What seems to happen is that there are (at the moment) two users who somehow managed to submit 2 usernames. If you look at Sentry's formatted HTTP post thing:

In the curl formatted thing you can see the data submission as:

username%5B%5D=noob-ceng&username%5B%5D=noob-ceng

(which unescaped means username[]=noob-ceng&username[]=noob-ceng)

Ultimately, it leads to an attempt to create the user twice! E.g.

IntegrityError: (1062, "Duplicate entry 'Cloppy' for key 'username'")

[peterbe]

1. How the heck did that submission come to happen!? - We need to review our JS code on the sign up
2. How come the server accepts it and attempts to save it twice?!