It seems that with some dopplegangers, the ONLY thing "wrong" with them, is that the real person is already home. This is especially true in the scenario you described.
I'm aware that happens sometimes, but in this case Steven was on the list of people who went out that day, and I had let him back in, so I figure there had to be something on that doppelganger I missed. Because like yeah, he was home, but I'm the one who let him in, so that type of doppelganger shouldn't appear for him? Or at least that's what makes sense to me, maybe the game actually does do that though.