Sign up: double _ not allowed

Hey!

I have a twitter ID as adam__moore and I generally try and use that as a public ID - I tries to create it on sign up and have ended up with adam_moore instead.

I'm not unhappy about this, particularly, but it would be good to check your username creation / validation to see if I'm doing something unexpected, or you have done something because of my rules (or I typed it incorrectly!)

Adam

The issue does not name a source file, test, or entry point. Start by locating the sign-up username creation and validation flow, then reproduce an attempt to use `adam__moore`; done means determining whether repeated underscores are intentionally rejected or preserving them is required, with coverage for the chosen behavior.

Written by the indexing model from the issue text.