Every click on group membership confirmation link sends an email

1. A user receives the double opt-in message to join a group with a confirmation link.
2. Click the link.
3. Confirmation message is sent.
4. Click the link again.
5. Confirmation message is sent again.
6. Repeat as much as you want.

Each link should only generate one confirmation message. Otherwise, we've ended up in situations where people end up receiving dozens of messages. Alternatively, confirmation messages shouldn't be sent if a user is already in a group.