It should exist only one "Page Not Found" page with "/page_not_found" path on concrete5 system.
However, you're able to delete that page or change the URL accidentally.
You can install it from dashboard, but it won't work correctly if you enables multilingual sections.
If it happened, sometimes concrete5 can't render 404 correctly and may occur an infinite loop.
I got a Segmentation fault error due to this case.
This job was written for re-installing "/page_not_found" page correctly.
Please upload this
restore_page_not_found.php file into your
then install it from Dashboard > System & Settings > Optimization > Automated Jobs page.
After install it, just push the "Run" button.