Is it usual that Google Adsense displays the same Adverts multiple times on one page?

Question

I have a Domain with two same-sized Leaderboards (728 x 90, Banners) and one Medium Rectangle (300 x 250, Block) on every Website. This answer says that

if you place more than one ad unit on a page, our [Google Adsense's] system will display unique ads to each ad unit.

Unfortunately the two same-sized Leaderboards often show exactly the same advert. Is this only related to personalized advertising or is this an issue of my code?

Answer 1

I found out that this adbehavoir is connected to or based on personalized advertising.