Let's say you make a time machine, and use it to go back in time an hour -- let's say from 3:00 to 2:00. For an hour, there are two of you, right? Now, imagine that, when time reaches 3:00 again, you both go back in time an hour. Are there now four of you in that time period, or just three?
It sounds so straightforward on the surface, but think about it: on the one hand, in the altered timeline you have created, there were two of you at 2:00 and now two more are going back to that point. On the other hand, where did that fourth one come from? The Time machine has only been used at 3:00, and in this timeline, it is only being used to send two of you back. So what do you think? Are there three or four?