Hello Graham, there are a variety of reasons why an image will play the same image multiple times. Here are some of them:
1) small number of images in the subcategory (which is 2 out of 3 duels)
2) even smaller number of unpaused images in the subcategory that are also close to the same imagescore
3) when new images come in they need to duel with live images that are close in score and they rotate though the available images. So the later duels are not so much to find the correct score of this users images but instead to find the correct score of new image coming in.
Since each of the voters are different we believe that every vote is adding clarity to the ImageScore. In an ideal world we would not have images play each other many times - but given the size of certain categories as well as other things that limit the pool of images that can be played - it is unavoidable.