code: def use_double_hash(self, password_hash=None):
          
          single_hash = current_app.config.SECURITY_PASSWORD_SINGLE_HASH
          if single_hash and self.security.password_salt:
              raise RuntimeError('You may not specify a salt with '
                                 'SECURITY_PASSWORD_SINGLE_HASH')
  
          if password_hash is None:
              is_plaintext = self.security.password_hash == 'plaintext'
          else:
              is_plaintext = \
                  self.security.pwd_context.identify(password_hash) == 'plaintext'
  
          return not (is_plaintext or single_hash)
docs: Return a bool indicating whether a password should be hashed twice.