Deluge
It seems that if you use 'Move torrent' plugin and have to readd finished (and automatically moved) torrents, deluge doesn't recognize them and starts downloading again...
Example: I have set in prefs to DL all torrents in dir 'A'. Activated 'Move torrent' plugin to automatically move finished to dir 'B'. OK, now for some reason (deluge UI crashes and I have to delete config files was the last one for me) you have to add all the torrents manually. Deluge doesn't now that it already downloaded some of them and moved them to dir 'B' and starts downloading them again to dir 'A'.