Preamble: I found a couple of threads related to this but not specific enough to simply reply. I also looked on Mantis but find that interface frustrating and user-unfriendly – if I do a search that has no results, it doesn’t even provide a “no results” response, and leaving users in the dark is bad design.
So, using phpList 3.3.3:
If a user unsubscribes and then resubscribes, they are sent a confirmation link correctly. However, upon using the confirmation link, they receive a message that their confirmation was already processed and nothing else happens.
This is false, their confirmation of the resubscription was not already processed. phpList is apparently “remembering” the confirmation of the earlier subscription, which immediately became irrelevant when they unsubscribed.
Prior confirmation data should be cleared out to allow for correct processing of the resubscription. This should not happen at the time of unsubscribing, in case the list owner needs (e.g., to answer a legal challenge) evidence of the confirmation, but it should happen when the person initiates the resubscription process.
Alternatively, confirmation data that predates the resubscription request initiation should be disregarded and overwritten upon confirmation of the resubscription. This, too, would allow the resubscription to be processed correctly.