Check for duplicate reply-to email address has been added on:
-verification endpoint, so we do not send the verifying notification
needlessly
- add reply-to email address and update reply-to email address
endpoints, as those can be hit multiple times after the email address
has been verified (so the same email address could end up being added
multiple times). EDIT: this has now been prevented on admin app,
but it's better to retain double-check for safety.