Every torrent have a hash id that identifies it, even if 2 torrents have the exact same files you are only seeding one of these hashes.

I regularly download mame updates by pointing the updated torrent at my ROMs directory. All changes are redownloaded, and new ROMs are grabbed.

Edit: just realized I misread the question. No. In my case, the previous mame romset swarm is usually abandoned, and the new torrent takes all the traffic. The swarms are unique.

What I described would be the ideal situation. I don't know of any existing P2P system that works like that, unless it's a system for sharing individual files instead of "torrents" containing multiple files.