Vuze automatically detects when two or more of your incomplete downloads share one or more files of identical size and will attempt to merge the torrent swarms to download the file faster or, possibly, complete an existing file with bad availability.
If merging completes successfully you will be informed via a popup indicating how much data was transferred between the file clones - likewise, if it fails, a popup will indicate that the merging process has been abandoned.
Well maybe not rare as you might think. Usually distributing sites add a signature file to a torrent making it a completely different torrent if identified by hash. The same thing happens when a torrent is repieced (same files, different piece size, so different hash) or reordered (same files, different order, so different hash). I would categorize this as a nice to have.