This can occur if you deleted or moved the file(s) from your default download location and did not delete the torrent(s) from the app. Each time the app launches, it will check to see if the file(s) are completely assembled. If they are incomplete or missing from the default download location, which will be the case if you moved or deleted the files, the app will begin to download the file(s) again.
In order to delete a torrent from your torrent list (once you have downloaded your file(s), go to your torrent list, then long press on the torrent. A trash can will display. Tap on the trash can, which will ask if you also want to delete the files. Select your option and then tap Delete.