In the Extra theme there’s a nice little feature called the Timeline. Most people do not seem to report a problem with it. But if you search in the support forum you will see a couple of people have had an issue with repeating posts.
They were advised by support to delete the offending post that is duplicating, in order to resolve the problem. The person that started the ticket was convinced there was a bug in the theme and I believe this was a fair conclusion.
Then when you use a Timeline page you’ll see the first post that was made, in other words the oldest post at the end of the results list, duplicating itself whenever you scroll down. You could force 20 duplicates if you did this that many times.
So strangely enough if you change the published date to something more recent, the problem goes away. The post that is now at the end does not have the same problem. However if you return to the original offending post and re-apply the original published date then the duplication issue returns.
Whatever you do the problem remains until you change the data of that original offending post. So if the issue is a bug then why does it affect one post and not the next. The answer is given in the date and this is why it only occurs in very few reported cases, myself included.
That date is the 1st of January. Make sure your first post is not dated 1st January and all is fine. This shows the issue is caused by a bug. If the date is a year removed then the post will not duplicate, it will not appear at all. For example your post dated 1 Jan 2017 will no longer display if you change the date to 1 Jan 2016.