record is potentially unchanged here, if so, no needs to save it

lib/devise/models/rememberable.rb

@@ -50,7 +50,7 @@ module Devise
       def remember_me!(extend_period=false)
         self.remember_token = self.class.remember_token if generate_remember_token?
         self.remember_created_at = Time.now.utc if generate_remember_timestamp?(extend_period)
-        save(:validate => false)
+        save(:validate => false) if self.changed?
       end
 
       # If the record is persisted, remove the remember token (but only if