Remember everyone, doppelgangers can read minds, including the minds of someone they capture before killing. So learning to act like you convincingly is more a function of time, and the fine details of manipulating your friends is then reinforced by reading their minds. Their biggest hangup is probably how lazy and selfish they are. They know what you'd do, they know how convincing their acting job is on your friends, and they may have even watched you, but they still aren't all that motivated to do it all the time. And I mean, hey, if they get caught then they just kill the person who confronts them if they can and run if they can't.

But yeah, a doppelganger would never pick Konsi to imitate for very long. She works too hard.

Honestly, if the doppleganger is acting exactly like Konsi, does it really matter if she got replaced?

Maybe they're not so much doppelgangers as members of a doppleorganization.