This option automatically removes duplicate posts from the page. The feature was designed to work on static pages. If you use pagination (including the Load more button and Infinite load) it will generate duplicates on the second page. For this last case, you have to use the offset feature. Read more about it here.
In the following picture you can see how two blocks which use the same settings display duplicate posts:
The Unique Articles feature can be enabled for each page, the option is located in the page editing section:
After you enable this option update the page and check the results: no more duplicate posts on the page: