(Sorry it took so long to reply, I couldn't remember where this was)
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I also can't remember, but I think there were two assassination attempts and the previous episodes, and I think the visit was after the first, and before the second. I'm pretty sure the Nightsisters took a lock of his hair in the first attempt - in the fight between several Nightsisters and Dooku - instead of the second - the threeway fight between Dooku, Savage and thingymawhootsit. I was wondering why they didn't just use the voodoo on him then. I'm going to have to look for and watch the episode online, since I can't be absolutely positive.
EDIT: I just watched the two fight scenes, and neither show an instance where a lock of Dooku's hair is collected, so they may have taken it when he visited Dathomir. But I didn't watch that scene, so I don't know. If they did, then that makes the second attempt superfluous.
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