From Magento forums.
By default, we notify you of the first one you haven’t read yet. So that if you looked at the comments after #2 you should have been notified of #3.
In your member profile email settings, there’s an option called “Enable Smart Notification” which lets you turn that behavior off. Incidentally, the forum notifications work the same way.
So we take that to mean that you only get notified once for each thread, if you have it turned on. If you turn it off you will get notified of every single post to the thread, whether you visit or not after you got the first email.
You could try it and see what happens, as you can alwasy change it back.