It is not the display name that has to be unique, it's the handle. We have the same issue with email, you get a "name" and an "email address". If the name reads "Bill Gates" but the email address reads [email protected] then you know it's problematic.

Is it trivial for the new user? Probably not. That is why organizations have training courses, where they tell the employees: Pay attention to the important parts (aka the handle, not the name)