I'm assuming you are talking about the Apps you use on your Roku and not the Roku device itself.
I'm going to use the Pluto TV app as an example. Pluto TV signs up for multiple different Ad services. Pluto TV is free and those Ad services is how they generate revenue.
Advertisers OFTEN purchase ad's from multiple Ad Services, but give each service the same Ad. When viewing a DIY/Home channel, they will try to show ad's you may be interested in. They may show 5 or 6 ad's during one commercial break, and they use 1 add from each services. There is a known issue where each service only has 1 ad related to DIY/Home so you will see the same ad repeated 6 times, or have to view the same ad's every commercial break.
I have also seen where the Ad service will also include foreign language ad's. This could be due to a lack of ad's in a category or I've have seen advertisers create a commercial in Spanish, but show it in English markets because even if sound was muted, someone could easily tell what the ad is selling. I see this happen mostly with Laundry Detergent commercials. They do this so they don't have to spend money on two different ad's