One of my site gets duplicate Adsense Ads. I am using the Leaderboard (728x90) and my first 2 Ads are from the same company, same text, same destination hypelinks etc. (confirmed it using Google preview tool)
Was just looking at one such example. Have seen it before and it is always the same company. Looks like a different ad only because one has DOMAIN.com and one domain.com but apart from that they are identical.
Usually it is because there are several affiliate pushing the same service/product. Many companies supply suggested PPC ads, and often affiliates won't customize them at all.
If you view the destination URL, you will probably see two different affiliate IDs, even if they are both going to the example.com domain.
Adwords does not allow the same company to have more than one ad show up at one time, although they do allow multiple affiliates of one company show up at the same time (through different Adwords accounts).