It would be pretty weird to argue that human lives decay in utility based on how many there are.
Maybe not as weird as all that. Given a forced choice between killing A and B where I know nothing about them, I flip a coin, but add the knowledge that A is a duplicate of C and B is not a duplicate of anyone, and I choose A quite easily. I conclude from this that I value unique human lives quite a lot more than I value non-unique human lives. As others have pointed out, the number of unique human lives is finite, and the number of lives I consider worth living necessarily even lower, so the more people there are living lives worth living, the less unique any individual is, and therefore the less I value any individual life. (Insofar as my values are consistent, anyway. Which of course they aren’t, but this whole “lets pretend” game of utility calculation that we enjoy playing depends on treating them as though they were.)
Maybe not as weird as all that. Given a forced choice between killing A and B where I know nothing about them, I flip a coin, but add the knowledge that A is a duplicate of C and B is not a duplicate of anyone, and I choose A quite easily. I conclude from this that I value unique human lives quite a lot more than I value non-unique human lives. As others have pointed out, the number of unique human lives is finite, and the number of lives I consider worth living necessarily even lower, so the more people there are living lives worth living, the less unique any individual is, and therefore the less I value any individual life. (Insofar as my values are consistent, anyway. Which of course they aren’t, but this whole “lets pretend” game of utility calculation that we enjoy playing depends on treating them as though they were.)