In order to provide a reliable experience on Strava, we limit the number of follows an athlete can maintain to 2,000. This limit is in line with the those enforced by other major social networks, and will help ensure better stability for Strava's services worldwide.
What's going on is that this athlete is following over 2000 athletes and every so often our system goes into their account and removes anyone that they followed after hitting 2000. Once the followers get removed, this athlete goes back in and tries to follow you all over again. This is why you keep getting notifications for the same athlete.
You can block this athlete so that you no longer receive these notifications as they will never be able to follow you unless they start deleting followers to get below our 2000 cap.