For a movie released in 2002, the visual effects are notoriously poor. The "Naag" (snake) looks like a low-budget CGI lizard. The transformation scenes are goofy. In the Tamil version, the dialogues often try to explain the bad VFX with pseudo-scientific babble, which makes it even funnier. It is a masterclass in how not to do VFX.
On public torrent indexes, the search term "Jaani Dushman 2002 Tamil Dubbed" yields results. Look for file sizes between 700MB and 1.5GB for the best quality. Avoid files under 300MB (they are unwatchable). full tamil duplicate jaani dushman movie best
If you are lucky, you might find an old MoserBaer or regional DVD titled "Jaani Dushman - Tamil Dubbed Double Disc." This is the holy grail of "duplicate" quality because it's an official (or semi-official) release. For a movie released in 2002, the visual