Forum notification does multiple notification, contrary to advertised behaviour

I was watching the thread below, and wandered off for a coffee.

While I was gone three messages along these lines appeared in my email Inbox:

A reply has been posted to a topic you are watching by Beakie.
View the reply at: Arduino Forum
Unsubscribe to this topic by clicking here: Arduino Forum
More replies may be posted, but you won't receive any more notifications until you read the topic.

They were at 9:59 am, 10:00 am, 10:04 am (my time, the times the emails arrived).

Now I was gone for around 10 minutes, and certainly didn't read those messages in that time. In fact according to the forum:

« Reply #24 on: Today at 09:04:19 AM »
« Reply #25 on: Today at 09:05:04 AM »

That's about 50 seconds apart. So the behaviour of:

... you won't receive any more notifications until you read the topic ...

is not working.

I have never had that - Normally this might occur if you visit a thread just after a reply has been posted. There is a slight delay between a new post and it appearing in your email (has been up to a minute or two for me at times) and then you might get two notifications close to each other.
From your post this is not what has happened but I suggest you simply un-notify from that thread and re-enable notifications and see if it sorts it. If it's only happened the once then there may be some other reason for it.