>> First, how on earth can I find this already existing torrent among many hundreds of torrents I have in my list?
Use "filter" option in the client. Finding your torrent will take nothing.
>>Second, what if the torrent already exits, what then? who cares? why should I bother finding
>>why mTorrent program prevents me from downloading the file again?
Because you already have that torrent downloaded. To do that multiple times delete torrent before next attempt.
So, my question How to get round this message of already existing torrent and just add the torrent again and start download?
Remove existing torrent.