The cache plugins save the output as a static html file and the visitors see the cached version.

PHP does not work and the visitors see the html file directly.

In this case, non-trailing urls are not redirected to the trailing slash urls. And the visitors see the cached version when both trailing and non-trailing urls are opened as below.

Trailing: http://www.wpfastestcache.com/blog/
None-Trailing: http://www.wpfastestcache.com/blog

Solution

WP Fastest Cache does not have such problem so you don’t need to do anything 🙂