Torrents can have the same files in the same spots, in which case, the client can match up 99% of a given file (1% needing to be redownloaded due to pieces that

Torrents can have the same files in the same spots, in which case, the client can match up 99% of a given file (1% needing to be redownloaded due to pieces that go across file boundaries). Alternatively, it can just have the old torrent file in the new torrent: Many clients support recursive downloading, it will then recognize that it already has the old torrent, and won't re-download.