The activity feed is a display of posts in the system and it's built in such a way that it only shows a portion of relevant posts at a time for easy viewing.
Posts are arranged by date, with the newest at the top. When a user scrolls down on the activity feed, the next set of relevant posts will be displayed automatically. In rare instances, a user may see what looks like a duplicate post when scrolling down through the feed.
This happens because the entire list changes when the next set of posts automatically loads. So if someone publishes a post, it will get added to the activity feed, which changes the order and can result in what looks like a duplicate.
Read more about the Activity Feed here.