OK.
Thanks for the clarification.
We are going to turn the stream off.
We only have a small community of 1000 people, so the activity stream is not very busy, and due to the way it works now it is driving people away from interacting.
If someone posts a new gallery and 20 people comment on the gallery, the whole page is just 20 repeats of the gallery.
If they are using a mobile device they have to spend a long time scrolling down to find a different topic/gallery/event.
This has caused most of our users to stop using our "Site Activity" page, which in the past was the most used page.

Using the comment bump does not work very well.
Only the new comment is bumped, not the whole conversation, so there is no context for the comment.

You say that this is the way Facebook works, but I have been in to look at Facebook.
Our town posted an article 2 hours ago.
It has had over 200 comments added.
I have been back to more than 2 hours ago, the article only appears once in my stream.
It moves to the top of my stream each time a new comment is added, I do not have 200 entries of the article in my stream.
  
I looked at other posts, I have no duplicate articles in my Facebook stream, no matter how many times people have commented or liked an article.
I have followed a thread that has been running several days, it is not repeated once in the stream.

I have attached an image from my Facebook stream.
Despite having over 600 comments, it only appears in my stream once. 
 

This is how the comments are appearing when they are bumped.