Deluge
OS: Ubuntu 14.04 LTS
Deluge version: 1.3.11
Current behavior:
When trying to add a torrent with the same hash (but not the same trackers) as a already existing one, first the add torrent dialog will show up as usual, but any settings you do there will have no effect. When finishing the dialog and adding the torrent deluge will silently try the merge the second torrent with the existing one by adding it's tracker. If the torrent is private, it will abort silently.
I think this behavior is undesirable. I think when adding a torrent with the same hash as an existing one the client should prompt the user if (s)he wants to add the tracker to the first torrent or simply ignore it. The prompt should also warn the user if either torrent is private and that such a merge may be against the private trackers rules. The add torrent dialog should not be displayed.
Ideally, the user should also be given the option to add the torrent as a separate seed (automatic cross-seeding), but this would be a whole new feature more difficult to implement.