module ActiveRecord::SecureToken::ClassMethods

def generate_unique_secure_token(length: MINIMUM_TOKEN_LENGTH)
  SecureRandom.base58(length)
end

You're encouraged to add a unique index in the database to deal with this even more unlikely scenario.
{validates_uniqueness_of}[rdoc-ref:Validations::ClassMethods#validates_uniqueness_of] can.
Note that it's still possible to generate a race condition in the database in the same way that

SecureRandom::base58 is used to generate at minimum a 24-character unique token, so collisions are highly unlikely.

user.regenerate_auth_token # => true
user.regenerate_token # => true
user.auth_token # => "tU9bLuZseefXQ4yQxQo8wjtBvsAfPc78os6R"
user.token # => "pX27zsMN2ViQKta1bGfLmVJE"
user.save
user = User.new

end
has_secure_token :auth_token, length: 36
has_secure_token
class User < ActiveRecord::Base
# Schema: User(token:string, auth_token:string)

Example using #has_secure_token

def has_secure_token(attribute = :token, length: MINIMUM_TOKEN_LENGTH)
  if length < MINIMUM_TOKEN_LENGTH
    raise MinimumLengthError, "Token requires a minimum length of #{MINIMUM_TOKEN_LENGTH} characters."
  end
  # Load securerandom only when has_secure_token is used.
  require "active_support/core_ext/securerandom"
  define_method("regenerate_#{attribute}") { update! attribute => self.class.generate_unique_secure_token(length: length) }
  before_create { send("#{attribute}=", self.class.generate_unique_secure_token(length: length)) unless send("#{attribute}?") }
end