Do values in the name column of the users table HAVE to be unique?

I've created a custom table to join with the users table. This custom table has first name and second name etc. I'll also be using the Email registration module meaning the user will not need to type in their username, so my question is, can I use the user's full name for the username in the users table? Though this makes it possible for duplicates in that column, is this bad?