With programmatic advertising, each time a reader clicks a page on your blog, each of your ad spaces undergoes a competition in which advertisers bid (think nano-seconds) to show their ad in that space.
Advertisers want to advertise to the right audience, so their bids are based on a couple things:
The content of your blog. Travel-determined blogs will get more travel ads than other types of blogs.
The specific reader's browsing history. If a reader has been gaming online, they'll suddenly see lots of gaming ads everywhere they go—unfortunately not always related to your content. Your readers are seeing ads that are targeted to them based on what they've been doing online, not your blog post.
It's totally possible that an advertiser might bid for, and win, every ad space on that page at once. This happens because they're willing to pay the most money at that specific moment and for that specific reader. It's a good thing—that ad space just sold for the highest possible amount.
The only way to prevent this would be to sell ad spaces at a lower cost, to a different advertiser who bid less for that space. However, having the same ad appear on multiple spaces is totally normal, and happens across all networks all the time.