Pulled from Reddit, but does a good job explaining how 2 versions of one person won’t exist permanately.
“ok let me try to make an example. So you have one of the reversal machines. You go into the red side and come out of the blue one with time running backwards. You can also reenter the blue side and come out the red side with time running forwards again.
Let's say you went to a swimming pool on a Wednesday, then on Saturday you went into the red side and reverted time around you, coming out of the blue door. Now you spend 5 days in reverse wearing your oxygen mask and see the world revert to the past, days passing in reverse Friday, Thursday, Wednesday, Tuesday, Monday. Now on Monday you go back into the machine from the blue side, coming out of the red. Time is running forwards again and you don't need the mask to breathe. You are now your future self from 1 week in the future back on the first Monday. Now if the future you goes to a roof of a building near the pool with binoculars (yeah pretty creepy) on Wednesday you can see your past self enjoying a day at the pool. You are in the same world at the same piece of time, but you are not clones of each other. You are the same person one older, one younger version. The younger version will get into the portal on Saturday and complete the loop, disappearing into the machine. The future you who is now out of the machine since 5 days will just continue as usual and from Saturday on there is just one version of you”
This means that following that although it looks like two Kats exist in the same world, the younger version will eventually go through the events of the film herself, culminating in her killing Sator on the yacht and jumping off, which completes the loop.