I'm working on an community homepage and 99% of the time a new user has an unique username. But sometimes it is required to allow duplicate usernames. Is there a way to change drupals default behavior from unique username+email+pass to unique email+pass?

I'm working on a Community Website and 99% of the time a new user has an unique username. But for my requirement, it is required to allow duplicate usernames. 

Is there a way to change drupal default behavior from unique username+email+pass to unique email+pass?