The recession is upon us. That seems to mean that for many topics in many locales, AdSense may display the same advertisement in more than one ad slot on a page. Of course, this reduces Click-Through Rate (CTR- the percentage of visitors to a page who click on advertisements) because nobody will click on the same ad twice, and people who are not interested in finding out how to get a flat stomach might be interested in finding out where to get gourmet foods. Variety of advertisements obviously should increase CTR. Google often puts duplicate ads on a page even when there are different ads (also duplicates) on other similar pages!
The ways that Google seems to use to decide what ads to put on a page according to content are somewhat mysterious. You may have a whole page about astrophysics, but if for some reason there is a single link to a poetry website on that page, they may put an advertisement for poetry there. There algorithm may be a bit primitive.
You would have thunk that if Google AdSense knows about your page content, they also know what advertisements they put there. Since Google gets revenue from the advertisements, they should be interested in maximizing click through rate, right? I have not seen that anyone who obtained an interview with a Google guru asked about this problem.
Until Google acknowledges the plight of their suffering publishers and fixes the problem, you can help fix this condition a bit. Different shaped ad slots will draw different ads (though large horizontal and vertical graphic ads seem to draw the same content. You can also specify that one slat accepts graphics while another is text-only. What a pity that Google has that rigid rule about three ad units per page, whether they are big ad units or little ones, and whether it is a huge page or a little one.