title: Allow duplicate E-mail for magento customer Registration
body: <p>My requirement, when the customer is registering E-mail exists validation is magento's default functionality, but we should allow the duplicate emails.</p>
  
  <p>I'm using a custom module for login with username (instead of email) into magento store.</p>
  
  <p>So your customization should support registration with duplicate email on front end registration/updation, back end(admin panel) registration/updation, importing from customer CSV file, And creating user from V2_API.</p>
  
  <p>Please Reply me with some good solution in detailed if possible.
  Any help is greatly appreciated. thanks in advance.</p>