Why does user column name has UNIQUE Constraint?
It's a bad idea, your are modifying a core feature of Drupal that others module (and the core itself) may relay on. That's the reason why that column has 'UNIQUE' option, because Drupal requires unique username. Think on user login: how you can login with different users if the have the same username?

Check this answer where you may find more info:
Is it possible to allow duplicate usernames ?