I am allowed to create two pages in the same space, whose key differs by case only, e.g. "Selector" and "selector":

But even though the links in the navigation tree are correct:


Both links lead me to the same page!

Workaround

Rename the page you can access and the second link will start leading to the other page.

Notes

It is generally not possible to create two pages with the same name (differing only by case) via the UI. However, it seems that there might some cases where this is possible which is what leads to the issue.