After installing WPML, I goto "Settings -> Media Translation" and click "Start" button, the process(only about 100 items to be processed) completes after several minutes, and then "Start" button will become clickable again. Then I click "Start" button again. These time there are 8000+ items to be processed.
And I notice that in the media library, there are many duplicates of the same image. Why?
It depends on the options that you've selected in WPML > Settings > Media Translation. By default, WPML will create database copies of the images(not duplicates) for all the media that you used on your posts, pages, and products for each of the languages that you're using.
If you open your media library, you'll see the language switcher on the top that allows you to switch languages. For example, one image will be copied over to each of the languages. However, in your server, there is only one image and not multiple same images.