The concept of unique listeners involves counting one particular listener, identified by the IP address + user agent, only once for any period of time.
With that in mind, if I were to listen to a station for 7 days in a row, a daily report would identify me as a unique listener for each of those 7 days. But when you would generate a report for that entire week (or month or any period of time), that weekly report would count all my sessions as one single unique listening session, not 7. Because those are not 7 distinct listeners over the course of a week, but rather a single one.
In other words, for any period of time, we are counting distinct listeners just once. If the same listener tuned in on multiple days, as long as their IP address and user agent remained the same, they would always be counted as the same listener and would generate one single unique listening session.