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Tell me something only you would know.
(comicpress.socksandpuppets.com)
These situations are a great opportunity to ask your party members questions that they normally refuse to tell you about.
That is a fair point, though. I've never had a Doppelganger impersonate a player's character before, so I hadn't thought of the impact of that wording.
I told this to my sister the other day and she had another good point in counter to mine: They may not be able to completely mimic the victim's memories, but they could read the thoughts of the person probing them to similar effect.