Imagine it this way. If you wanted two copies to act the same way, it's like a science experiment. You need the exact same variables, so if even one small thing is out of place in your copy's life, it will not live the same life.
So if you and your copy both lived at the same time on two copies of our galaxy in the exact same circumstances, you could expect the same results.
But if you both had different thoughts or different brains, it wouldn't work. I believe if someone had your exact molecular structure, they'd be the same person.