Can we have unique identifier as an email-id for an user??

Hi,

As per current situation for every user we have, username as an unique identifier that should always be unique and also same is used as login id & Email id is an optional field. 

As per our requirement,

     1. Email Id should be unique for every user ( no repetition should be allowed) and should also be used as secondary unique identifier for every users.

     2. Also want to use email-id as a login-id.

How should it be implemented in community??

What are the optional proposed solutions??

Thanks