I am nowhere near this part of the campaign as we technically haven't even started The Whispering Cairn yet, but I have been trying to figure out how I am going to handle it. My group are gamers first and role players second and when we are playing pathfinder we tend to be more of the "roll" player types and less "role" player types. I say all this because one of my groups favorite games is Battlestar Galactica. For those who don't know Battlestar Galactica uses a hidden traitor mechanic similar to Shadows over Camelot or Resistance. I mention all of this because I was thinking of bringing the Battlestar concepts into my AoW games concerning the Doppleganger.
1) When my players first encounter the Dopplegangers (most likely in the crooked inn), I will inform the players that one of them has been replaced by a doppleganger and that I have secretly let that person know. This will however be a lie and I will not have told anyone that they are the doppleganger. I believe this will ramp up the paranoia within the group to the appropriate level.
2) When the players reach the area where they meet their doppleganger selves, I will tell them that I didn't mention to anyone that they are a doppleganger yet. I will then pull out 1 "You are not a Cylon" card for all but one player and 1 "You are a Cylon" card from Battlestar Galactica. I will show them to my players and tell them that if you get the Cylon card, you are a doppleganger. I will then shuffle them, deal them to my players, let them look at them, and then take them back one by one so that I know who the doppleganger is.
What do all of you think this? Do you see any sort of issues that may come up? Any other ways you think to handle this?
I think it'd definitely be different than what the book envisions, but if your players won't just kill each other out of suspicion, it could definitely be fun. The biggest difference would be that, rather than having an active saboteur, you're letting them knowingly sabotage themselves through arguing. Obviously it makes a letter less sense from a narrative perspective, but if your players do lean towards the "roll" side of things it shouldn't bother them.
I'd say it'll achieve the desired paranoia, but without requiring someone to act. If that trade-off seems right for your group, go for it!