I honestly believe that there is a finite amount of matter that exists in the universe, and this cannot be increased or decreased, only manipulated. Extrapolating from this, I assume that given the reality of time travel, it would be impossible to travel within the same universe as one would either increase or decrease the amount of available matter in that given universe. For this reason, I believe that it would be impossible for an individual to be in the same universe, so an individual and his/her time travelling duplicate could exist in the exact same place and time, but in a different universe or dimension. Thinking this way has allowed me to think about time travel in depth, and write a few bits of flash fiction, without running into paradoxes.