Yeah, this is something that's been bugging me about time-loops in SF for a while:
Suppose you are given a gun by your future self that has six bullets in it. You use one bullet to shoot Hitler, and then travel back in time and give it to your past self.
Presumably, your past self now has two guns. And assuming a time-loop, he will use the gun with 6 bullets to shoot Hitler - if he used the one with 5 bullets, he'd be breaking the loop.