A few questions to that.

1) Why would you want to download a file if you already have it?
2) Do you mean you have an incomplete one, and you click on the torrent link to download it, and the incomplete file restarts?
3) What bt client are you using?

If it's question number 2. Then i have but one answer =P
Don't click it. You can't download from more than one tracker at a time for each file. If you do, it either starts a new one, or even worse (depending on your client), overwrite your existing one.

Each torrent link uses a different tracker depending on the fansub who did them. If you suddenly change the tracker, the file loses it's place because the new tracker has different IP addresses and references than the other tracker you used. So it dosn't remember how much it downloaded, and from who, so it has to start all over again.

So for each torrent link you click, make sure you choose which fansub & website you're going to use, before clicking them.

(Yes even the website. Some websites might make or use a different tracker for the same file done by the same fansub. This is the result of reseeded files, and an up to date list of IP addresses for the torrent to look at.)