To clarify, my suggestion was that it might have been so long ago that he has no clear memory of his birth name -- it sounds vaguely familiar when he hears it, but he doesn't know why. And of course, there could be other reasons he doesn't remember his birth name, like amnesia or a memory wipe. Or maybe he's only pretending not to recognize it. There are many hypotheses that can work at this point, and insufficient information to rule any of them out. But at this point, I think the doppelganger hypothesis has a higher probability than the hypothesis that Sarge is somehow only a year old and a reincarnation of "our" Coulson or something like that.