In Kurokami, if your luck higher than your doppelganger, the unlucky one will be dead next time by accident or something and vice versa.
1. A look-alike isn't you though, it's just a total stranger that happens to look like you. 2. I'm pretty sure it was a main-sub deal and not just random chance whether you or your double died.
The lack of reaction from either vanilla Jeanne or Jailter suggests that yeah, yeah, they've already seen this shit from Jalter before. Because of course they have.