The only issue is that we're looking at creation and/or destruction of matter, undermining a major law of the universe. Can matter exist in two places at the same time in a non-theoretical way (as quantum uncertainty has things existing in multiple states simultaneously, but that's only because we lack the ability to perceive energy and position at the same time).
If going backwards in time is at all possible, then i suspect it would result in a duplication of matter. There would simply be two of you, and if you were able to prevent your past self from making the decision to go back in time, then there would be two of you in the future as well, but if you do something in the past that prevents you from ever coming into existence, that doesn't impact the you that currently exists.
Monster Hunter: pissing me off since 2010.
