I’ve noticed some affiliates put several banners for the same service on the same page. Putting two banners featuring the same offer on the same page won’t increase you chances at making sales, but might instead overkill it, even if these banners are in different formats. Banners being highly noticeable, I would instead suggest combining them to text links, if you want to promote the same offer on a couple different locations on your page.
This works for Adsense as well. Putting several copies of the same unit on the same page will create some kind of saturation in the eyes of the visitors.
That is why, instead of thinking of affiliate programs and Adsense being competitors, you should instead see them as complementary offers and sources of revenues: combining affiliate program ads and Adsense units is indeed the most effective way to maximize your online income.