Which is why it's good for marketers but not the user. I don't need to see the same ad more than once in a session but many video sites will display the same handful of ads every couple minutes. This is especially prevalent on twitch.tv when watching gamers stream, it's usually the same ad every time. After the first time I see it I'll just visit another tab while it's playing (mind you the audio is still playing.) If they were to pause while I was doing this I wouldn't bother watching at all.