What SonicSam plagarised
is completely correct. Just leave it downloading (and uploading). Eventually it will finish.
Another possibility is that part of the torrent is a system file (eg Thumbs.db). Your computer keeps overwriting this this file, and the BitTorrent client downloads it again, but the computer overwrites it again. When your client checks the file, the hash doesn't match the one that it should be, the client delcares it not downloaded, and you're back downloading that file again.
Try using the download even though it says it is incomplete. Maybe it is usable already, and the client is just stuck redownloading some files that are removed or overwritten by your computer.
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