When adding listeners to a feed, they'll be assigned a link that's unique to them for that feed. The listeners are identified by their email address in the Listeners & Links area of the feed.
Right after being added, their status is set to "Awaiting Activation". This means their unique link will lead to a subscribe page when clicked.
This listener's unique link is enabled and they can subscribe to the feed.
Once they subscribe to the feed in an app of their choice, their status changes to "Activated". This means their subscribe link is no longer active.
This listener's link is now disabled. They can continue to listen to the feed, but are unable to access the subscribe page as before.
If they try accessing the link again, they'll receive a message stating "This feed's subscription is already in use. Contact the feed creator to reset your access to the feed."
This is a privacy measure to prevent listeners from sharing links broadly for many to listen without permission.
Allowing Listeners to Subscribe More than Once
If a listener contacts you saying they would like to subscribe again -- usually because they have a different device they'd like to listen on -- you can find the listener's email address in your Listeners & Links area and click the "Resend Email" button. This changes their status back to "Awaiting Activation" and their link becomes useable once again.
After they subscribe this next time, their status again changes to "Activated" and the link can't be used to subscribe.
This single-use functionality is only for unique links of individual listeners. It does not affect the feed's Universal Link. The Universal Link remains usable until a new one is generated or the feed is disabled.