Add the following to the site/override/site.ini.append.php
[UserSettings]
# Authenticate match, a list of authenticate fields to use
# Available are login and email
AuthenticateMatch=login
# Controls whether a unique email is required for all users,
# if set to true and email is set in AuthenticateMatch then only
# one email address instance is allowed on the site.
# Set to false to disable it RequireUniqueEmail=false
