How to work around “that email address has already been used” error?

The following error may occur if a previous registration remains pending with a conflicting email address.

That email address has already been used. Please check your inbox for
an activation email. It will become available in a couple of days if
you do nothing.

The related username error looks like the following.

That username is currently reserved but may be available in a couple of days.

1. Login as WordPress administrator
2. Navigate to add a new user
3. Check box to bypass email confirmation
4. Add the user with a different email address
5. Edit the user’s email address to the desired result

For the similar username error, see How can I un-reserve a pending username registration?

The timeout period is 2 days following a conflict, and trac tickets to clarify this process currently exist here and here.