When a user unsubscribes a second time, help them know

[jace]

A user may receive multiple emails via a list and then choose to unsubscribe by clicking on the unsubscribe link in each email.

A list manager like Mailchimp will assure them each time that they have been unsubscribed, even if they already are. This is not particularly reassuring to a user. To fix this UI pattern, the unsubscribe link must be a signed token that encodes when the email was sent. The user can then receive a more informative message:

"You are already unsubscribed. This email was sent to you on XXX date, before you unsubscribed."