I can add any 3 number variations on the end of link

I have a multisite setup with subdirectories and have noticed that I can add 3 numbers to the end of any link. For example domain.com/about-us/123 this doesn't go to a 404 it just returns the same page. Would you have a any clue to why this would happen?

I'm concerned that this might be duplicating content and therefore want to do something about it quickly.