If you have instant notifications set for a thread, you will get one notification when any number of posts are made there by a given user. It's not like a daily notification where you get them once and that's it.

So:

User 1 makes a post: you get a notification
User 2 makes 2 posts: you get a notification
User 1 makes another post: no notification
User 3 makes a post: you get a notification
You read the thread
User 1 makes a post: you get a notification
User 1 makes another post: no notification
User 2 makes a post: you get a notification

etc etc

You may want to change the notification type for the threads you're getting multiples from, to daily or less.