If he doesn’t see someone at least every few days, they cease to exist or, at the very least, he forgets their name, forgets where they met, or why, and whatever they may have once meant to each other. He’s not proud of this, but accepts and readily admits it. Doing so decreases both his guilt and embarrassment, as well as the shock and dismay of those whom he encounters.
It’s no great surprise, then, that when he meets him on the street, after having become estranged almost two decades earlier without so much as a phone call having passed between them—even when his mother died—he has no clue to his identity beyond noting an uncanny physical resemblance.
One might have hoped for a somewhat stronger bond, a clearer recollection, especially for a brother. Especially for an identical twin.