Specifically when it comes to Cavendish and Ray
As far as I can figure, the two were essentially contemporaries. Cavendish and Ray were probably born around the same time. So why do they both have the birthmark? I thought that the comet soul was reincarnated once in every timeline, but it would have been reincarnated twice with Cavendish and Rey.
What am I missing?
You're my favorite person, but every once in a while, you can be a real CVNT
Up to you to decide. My personal interpretation of "reincarnation" in the movie isn't "Oh, I died but it's cool because I get to live again in a different body!" It's that we are all influenced by the past and also in some way affect the world that follows after us. The comet is just a story device to show how the story that begins with a man on a beach is the start of a line of successive lives that ends up on another planet. The last image of the movie being an actual comet, stirring my imagination to wonder how far the story can go. As I said, that's just my personal interpretation.
Some people like to cite it as an error but the movie made no promise of the nature of the comet birthmark, so holding it to a more traditional view of reincarnation isn't really fair. A more poetic view I've heard before is that the reincarnation of souls transcends time.