What will I do if more than one user has the same name?

Low-code
#1

In Account, there is no field for Name, only username field, and this is unique, what will I do if more than one user has the same name?

#2

Do not use username for the name. Username is a unique string that is needed for login with a password.
There is an owner field that has other properties that can be used e.g. for the name.