How could you know what a perfect simulated version of me would do in advance, without first running the code? Even if you found a more efficient algorithm for getting the exact same end states faster, at some point you will reach a computationally irreducible limit, where you can’t predict what the improved simulated agent will do without first actually running the code. When you run the code it will freely decide to do one thing over another, and no one can know what this will be in advance, even in principle.
And even if we make an exact duplicate of you, it would immediately start to diverge from you, as it started accumulating its own experiences.
The rate of divergence would not be steady or linear. Particular events would accelerate it or lead to sudden insights or changes of approach for either of you.
Stating there is no free will does mean we can predict everything you'll do. The only way to simulate the universe is to produce another universe and even then the reality of quantum physics is that future states have randomness not perfect determinism. That said, saying you have free will is saying you have some "magical" extension of self beyond the physical.
> Stating there is no free will does mean we can predict everything you'll do
You can only predict it after you've run the simulation once, and after you have let the simulated person do what they want to for the first time. During the first run, free will is unpredictable.
Only after you have run it can you say "if I run the simulation again with the exact same parameters, I know what they will do next".
And if you find a way to create a faster version of the simulated person algorithm, then the problem just devolves to the faster version. How can you predict what the fast-free-will person will do before you let it do what it wants to do? And so on.
