Account migration notifications are duplicated
Haven't seen it being reported here before, but it seems to be a long-standing issue. It seems like the notification is sent out to each follower N times, where N is a total amount of followers on the receiving server the migrating user has.
A couple of recent examples I found:
https://clubcyberia.co/objects/3b44fece-7fac-49e8-8b10-5597df5e6be7
https://poa.st/objects/49e0c46b-b75c-41d1-b4b0-59c94d24540b
https://bae.st/objects/73caf044-1e14-4eb1-bf95-268c3246a199
Edited by mint