Duplicate mongo entry error on second put

[mmpadget]

From Heroku staging:
{
"name": "MongoError",
"message": "E11000 duplicate key error index: heroku_ttts92xp.users.$username_1 dup key: { : "ben" }",
"ok": 0,
"errmsg": "E11000 duplicate key error index: heroku_ttts92xp.users.$username_1 dup key: { : "ben" }",
"code": 11000
}