Recently I shared a large upload - 350+ files as a complete collection.
The size of the upload is 165 GB
But now that I have uploaded less than half of it, there are several seeders, who (I guess) have the full package.

I am just wondering how that is possible?

Not all the files in the collection are unique - some of them can be found in other torrents. So is it possible that the system is clever enough to merge other sources into the torrent I uploaded, so leechers can get the files from other sources than mine, if they're identical as part of the full package? And therefore quicker become seeders themselves, even when they didn't download all of it from me?

It would be clever, I have to admit,and save a lit of bandwith…