As Appogee Leave uses email addresses as a primary identifier, they will need to be unique in order for a user to be correctly identified, as such, email address sharing and email address re-using is not supported.
Should an admin wish to re-use an email address, the new user re-using an old email address would be able to view requests that were created by/for the old user.
As such, we highly recommend using new email addresses. If using a new email address is not possible, we recommend first exporting the old user's requests and then cancelling all their requests to restore their allowances. The new user can now use the request with a new balance.