In Season 4, episode Acceptance. Hiro repeatedly tries to save a jumper who is on top of his building at work. he keeps coming back to the starting point (getting the call) and then going upstairs to try and talk him down. He does this numerous times. This is perhaps the only time we see Hiro return to the same point over and over again at the exact same moment in time.
Is Hiro replacing himself everytime he starts over? Otherwise he should be running into himself each time he goes back.
Clearly it is possible for two Hiros to exist in the same time, because the original future Hiro came back to talk to Peter (while the current Hiro was in Japan).