I serve 3 Adsense ads on most pages on my site; same size ads and format. It's actually the same code in 3 places. I frequently see the same ad/advertiser displayed by Google. Does anyone experience this?
I only see it if I have two blocks of adverts on a page... BUT, they belong to separate accounts. It may be different if they were the same account. I don't know. I never use more than two, and mostly use just one per page.