In short any advertising network that really knows what CPM is will not count visitors unless they are unique. CPM means cost per mile, where mile means 1000, that means CPM actually means cost per 1000 visitors and hence every visitor should be unique.
They could show a different ad for each impression, giving different advertisers a chance to show their ad
Right but one user can see several impressions depending on how many times they refresh the page (rarely) or return to the site later. We measure impressions by how many times an ad itself is viewed, not by how many people view it..... or at least tapit does. to be as clear as possible: If you open a site and an ad loads, that's one impression. If you hit refresh ro navigate away and come back to the same page, the ad has counted two impressions. Im sure you know how it works n i dont mean insult by explaining it so, but i wanted to limit as much confusion as possible before explaining the following: We (and most ad networks) pay per 1000 impressions (per mille) and we base impressions on the above description. If other companies do it differently im unaware (or just naive)