If you are seeing duplicate posts in your listing, especially in combination with a Pager/Load more facet or Sort facet, the cause is almost always the way the posts are ordered in the query, which is determined by the orderby query argument.

If results are ordered by a value that is the same (or empty) for multiple posts, or by a random value, MySQL does not have a fallback and will often sort erratically.

In this tutorial you can read which scenarios can cause this, and how to fix it quickly by adding a secondary, fallback sort order to the listing query.

See also