it's normal..... sometime some ads show up more than 1 in a page. coz, the advertiser used many keyword to place their ads on the netwrok. or theres not enough ads inventory to serve in your page.
It is perfectly normal. A number of times I have same ad on many of my pages and even same ad more than ones on the same page.
I have seen duplicate ads in sites that need keyword optimization to get displayed different, relavant advertising.