I just experienced something similar - although my upload was several files in one torrent.

Was yours just a single file or more files in a compilation?

https://forum.gaytorrent.ru/index.php?topic=56744.0

My guess is that most people who take their time to contribute with uploads do so with the intention of sharing something for other people to enjoy.
And yes, it may happen to be a duplicate. Some torrents are uploaded under different titles, and there's a chance you won't find the original upload, even when searching quite thoroughly for it before making an upload yourself.
If it were to be punished making a mistake, it would probably also deter people from attempting to contribute

Hi;

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…