[dTal]

>Surely in this scenario the answer to "is it OK to kill one of you now?" is "no".

I don't know about that.

I'm not sure about this intuition pump. If they are truly identical to me, then the world they live in must also me identical to mine or we would have diverged. So if we discovered Earth-2, then Earth-2 would simultaneously discover us (or Earth-3 etc). No actions can cause divergence, so it's not possible for anyone on any Earth to only kill one of you. It would be like trying to kill your reflection.

If we're talking about some kind of god who is outside the mirror/chain Earth system, and has the ability to break symmetry as they see fit, it's still not clear-cut. In effect, from such a perspective, there is only one Earth (albeit with copies) until changes are made. It's not even clear that deleting one of the several identical copies of the whole Earth is wrong. Moreover, deciding to kill someone on one of several heretofore identical Earth's and seeing how they diverge is informationally the same as spawning a new Earth - an act of creation rather than destruction. Nothing is lost - that person still lives their life on an Earth somewhere - but now new lives will be led, new experiences, new sadness but also somewhere new joy.

I think it's pretty clear though that our morality of "no killing" is based pretty strongly upon the assumption that there's only one of someone, they can only be killed once, and it's forever. Start violating these assumptions and it breaks down quickly.

[norea-armozel]

Only if you assume the finality of death makes anything less good. If you assume every conscious being has inherent value then the entire argument has to go another way which isn't much better, but it can be characterized as a viable alternative if perfect copies of any one person could be created down to the Planck constants.