At registration of new users e-mail addresses are checked on duplication. If the user is already registered, and has decided to replace the address in the registration data - check on uniqueness is not carried out.
It means, that on a site there can be some users with identical e-mail addresses
If a user tries to change their own email, it won't allow them to set it to an email that is in use.
It is possible to set duplicate emails with the user admin panel. I believe this is by design.
