This actually happened to me, about three years ago. At the time, I was working at the Royal Museum in Edinburgh, Scotland, classifying dead seabirds. They would bring in the corpses from the latest natural disaster, covered in oil or chemical spillage. I would examine their plumage, briefly decide on cause of death, and add the results to the statistical database whilst making comments about the museum getting more shags than I did.
So one day, I think it was a Thursday, I was sitting at my lab bench as usual. Tony the message boy brought up a cardboard box, and inside was a remarkably well-preserved Arctic Tern corpse. I washed my hands and laid it out on the table, but inspecting front and back I could see no cause of death. I was about to head off to KFC for lunch when I heard a strange buzzing noise and this slightly disoriented man walked in.
After a few seconds, I noticed the resemblance. His hair was close-cropped, not like my straggly mane. His eyes were marked by fine wrinkles and he looked thinner, gaunter, tireder. But he was exactly my height, and there was something very familiar about him.
"Who are you?" I asked him.
"I think you already know."
"But you can't be ... what are you? Do I have an older brother?"
"I'm you," the stranger said, "twenty years later. I've come back to warn you."
"How do I know you're me?" I demanded.
"Don't you remember. When I was a child we made a password for this occasion. Devonian chicken slurry."
I looked at him. I did sort of remember a conversation about time travel and identity, and yet, now I was a real scientist and not just a kid with too many Robert Silverberg books, I had to take such things less trustingly.
"You must have overheard me. Maybe I talk in my sleep."
"You know you don't do that."
"How can I possibly know I don't talk in my sleep?" I asked him.
He was stumped. "Ok, let's think about childhood memories," he said. "Remember how you buried the goldfish under the cherry tree."
It still wasn't proof. Maybe he was friendly with the dog that dug in the garden. Maybe he read my mind. But we had facilities in the museum to do DNA testing. I took a blood sample from each of us and told my double to hide in the supplies cupboard. He was muttering about having no time to waste, but I said he'd come 20 years and a couple of hours wouldn't matter. As a concession to his worries, I locked the lab door.
The test results seemed to confirm it. We shared the same DNA, or at least I couldn't find any differences between us. Results that were a trillion to one against. I returned to the stranger to give him the news.
"So you have the same body as me, as well as the same mind," I said. "Assuming I can trust the test. For all I know you're a clone of me aging faster due to cellular damage from the cloning process. Or maybe I'm a clone of you. Ew."
"We have the same mind," he said. "Cloning does not explain that."
"Actually the same mind thing proves nothing, because if someone created you with some kind of matter duplication ray and you have an identical body it is likely you will have an identical brain. And possibly vice versa." I considered more hypotheses. "You could be an evil duplicate of me sent back in time by a spurned lover or a scientific rival or a great crested grebe angry at finding his great grandfather spent his last minutes on my dissecting table."
"This is ridiculous," the interloper said, his patience snapping. "See that bird on your dissecting table. It is infected by a horrendous new virus that will wipe out the human race. Unless you utterly destroy it now, all humanity will be reduced to a few hundred scientists scrabbling in research labs trying desperately to find a cure."
"On the other hand," I said. "If I spend any more time talking to you, I'll get fired from here, that'll seriously damage my chances of ever getting more lab work or someday an academic position, and I'll never even have the slightest possibility of being part of a scientific program aimed at combatting mysterious bird viruses." I weighed things up. "Either way, I have to bid you farewell."
He got angry at that point, but I had a scalpel easily to hand. I stuffed his body in the incinerator and got back to work.
On my way home I chuckled at his warnings, thinking, "Hey, I'm not dying of some bird disease." I got off the bus and walked up to my flat. In the stairs 47 of me stepped out of the shadows and started bludgeoning me with baseball bats, rolling pins and metal bars.
I lapsed into a coma for three months, and when I awakened I could remember nothing. That was the point at which I realised my future self had been full of shit.
Thanks for all the interesting comments. ariels said: "YOU FORGOT TO ASK HIM IF HE WAS YOU *AFTER* YOU'D SAVED HUMANITY FROM THE VIRUS OR BEFORE! (Trick question!)". That's one worth remembering. Rose Thorn said: "The only question is why you went back to tell yourself that crap, knowing that you would kill you." Which is also a valid point. I also had a couple of people asking if anyone did kill the bird; um, my memory's a little foggy on that issue.