“post order by” random doesn′t work correctly

Hi,
i am having difficulties when using random order by. I am using the following shortcode.
[bdp_post design=”design-2″ grid=”2″ show_date=”false” show_author=”false” show_comments=”false” show_category=”false” content_words_limit=”8″ category=”16″ orderby=”rand”]
I have about 20 posts that are listed on the first page. When I switch to page “2”, there are about 12 posts that were already listed on page 1. So I have 12 posts that are the same as on page “1” and only 8 new posts.
On the same side there are 38 posts shown on page 1 and 2 all together. That is the exact number of all posts. But since most of these posts are repetitive in the post list, there are not all posts listed in your post list.

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