sometimes (or some people, like me and many other "harvesters", when you explore a topic, and seek using tags and keywords - you download a lot of files "mechanically" in order to check them later. I don't count my downloads, I just click and go next - in order to not forget what I have in my mind. even with a good online preview I would do the same, because I go back to downloaded material some hours or days later, to hear it from better perspective.
sometimes, after download, you reorganize your downloaded data (unpack, rename or delete files). well - pretty often, after that - when you explore again a similar or same topic, or just because files have many different tags - you begin to re-download the same files, without noticing it. how many times? sometimes even 10 times per year (-;
sure, you have on your account "files you have downloaded" list, but that list is without any context. I'd like to see on each sample page (and pack page) an expressive indicator, that I have already downloaded this file (very optionally - how many times with last download access date). you save the bandwidth, registered users save time and even more.
downloaded indicator could also appear during the search (next to samples), so it would release the sample page reloads and the database.
should be easy to implement.