This works around a WebTorrent bug where calling client.add(torrentFilePath) to add a duplicate torrent -- in other words, one whose infoHash we're already torrenting -- creates a new torrent object and later throws an error. Inconsistently, calling client.add(magnetURI) or client.add(infoHash) to add a duplicate torrent returns the existing torrent object that we're already torrenting and doesn't throw an error.
This also fixes a prety nasty bug where pasting a dupe magnet link changed the torrentKey of an existing torrent, breaking the communication between the main and WebTorrent windows
Fixes#364
0b85ba9f32