Adding multiple slashes after the site URL breaks the current entry, but still loads the template
Are you getting the About template or the layout template it extends?

In a test with a local site I have running, I was able to trigger a 404 for any bad requests by putting 

{%  redirect 404 %}


inside the main block in the _layout template.

This works in my case because that block is overridden on every request. If it's not, something has gone wrong and I need to exit out since the layout template should never render on its own.