Transmission

This happens to me ALL the time and TM seems to be the only client out there that has issues with this:

1) Download something with TM
2) Quit TM
3) Delete file you downloaded
4) Start TM
5) TM starts downloading the file again! UGH

Can you make TM a bit more smarter so it knows that it already ...

Generally, this is a bad thing and there are many reasons why uTorrent, Azureus etc disallow this behavior.

You should really complain to the torrent tracker about banning clients.