Is it possible to register duplicate email addresses when registering as a user?

You cannot register duplicate email addresses, but there are ways to avoid duplicate email addresses.

If you register an already registered e-mail address, "This e-mail address is already in use" will be displayed.

As a workaround, it is possible to add "++" after the e-mail address (user name) and add an arbitrary characters.

For more information, see the link below.