CI 3.X Session Core problems

My program occasionally appears with the following error：
Fatal error: Class CI_Session_files_driver contains 5 abstract method and must therefore be declared abstract or implement the remaining methods.


My configuration file is as follows：


I've been searching the internet for a long time and I can't find a solution.But I go to the CI of the following system/libraries/Session/drivers found the problem。

This is Session_driver.php


this is Session_files_driver.php


Why implement this interface twice?I felt this would be a fatal error.I think that's why I've got a fatal error in my project.

May I ask a CI developer to tell me why? Thank you.