Creating a wiki page named "pages" causes page to become unreachable

Summary

It is possible to create a new wiki page with the title "pages", as this name is the same as the directory page it will become inaccessible.

This bug could also effect other default pages.

The page is also reasonably difficult to remove, requiring the user to modify the browser URL to navigate to the edit page.

Steps to reproduce

Create a new wiki page with the title "pages".

What is the current bug behavior?

Two pages called "pages" exist, however it is not possible to access the user created page.

What is the expected correct behavior?

The wiki should not allow for two pages with the same name to be created, therefore it should report an error when creating the page

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Possible fixes

Have a sanity check that the page title is not "pages"