Banners must be shown unique on one page (Joomla website)

For a Joomla website there is a problem with showing banners between articles. The banners are shown and display random. The problem is that the banners are not shown unique. Sometimes the same banner is shown multiple times on the same page. For showing the banners I use the extension RS Banners and a code {BANNER} after each (ZOO) article. I need the banners to be shown at random (already applied) and unique (the problem). The maximum number of articles on a page is 8 articles and there will always be 8 banners active. Who can help me out? You can find the website by following the next link: <[login to view URL]> Greetings Viktor