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¿Question #228377?
Woelf: If a duplicate of yourself arrived at your doorstep and claimed to be you from the future, what could they say or do to prove it to you with 100% certainty? (Assume a stable time loop.)

ThroughTheDeckGlass: Fingerprint, retina scan, & DNA match for me-ness. And exact same freckle pattern, and predicting my thoughts & writings. Predicting specifics of the stock market, lottery numbers, and some gamble-able sporting event for the from the future-ness. And predicting odd world events, people's death dates, etc at least 10 times. Or, take me on a trip to the future.
TheGrimReeple: Once we've moved beyond known science enough to allow for time travel, nothing I can think of would convince me 100%. Physical resemblence could mean a clone or doppelganger. "Tell me something only I would know" won't expose a mind-reader. Deranged's trick with the note wouldn't rule out the possibility of the note being stolen in the future, or the time traveller showing me something like Doctor Who's "psychic paper" that would trick me, or a telepath manipulating my mind to make me write a copy of the note he's already carrying. The more tests they pass, the more convincing they are, but never quite 100%. So I'd go for all of the above, plus a couple more. Grab a few dice, have her predict the roll. And compare fingerprints (as I understand it, that's one of the things that differs between identical twins, so it stands to reason that it could also potentially identify a clone).
Derang3d: Hmm well with a stable time loop I could ask him politely to wait half a minute so I could write a random sentence on a piece of paper with a drawing of a random object on it, fold it a few times, put it in an envelope, put the envelope in a book of my choice, and then open the door to have him hand me that exact piece of paper - which I can then cross-check with the one I just put away (as long as I remember to put it back again after). Still not foolproof, but it gets close.