I have the same few npubs repeatedly following me. They should not be able to inflate our follower numbers, though, as the same npub should only be counted once by clients attempting to figure out how many people are following us.

There's not a list somewhere of who is following us and each time they follow, their npub is getting added to that list again. That's not how following works.

To try and calculate the number of people following you, a client has to request all of the follow lists that contain your npub from all of the relays the client is aware of, exclude duplicates, and then tally them.

I'll keep an eye on this and see if Primal is adding them to my total number of followers each time they follow me. If so, then that's a bug in Primal, since they should exclude duplicates.