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

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.