How to disable a customer registering with the same email address into the website ?

Customers with different passwords, even if they have the same email address, should be able to login to their specific customer centers.

Duplicate Email Detection, that prevents web store shoppers from creating duplicate customer accounts.

To enable this feature:

1. Go to Setup > Company > General Preferences.
2. On the Web Presence subtab, check the box next to Web Site Duplicate Email management.
3. Agree to the terms of service.
4. Click Save.
5. Go to Setup > Company > General Preferences.
6. Under Web Site Duplicate Email management, select Enable Duplicate Detection.