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My understanding is that for us to confirm that a signal is of artificial origin, we have to get the same signal at least twice. However, as far as I can tell, most of the signals we've sent out to contact extraterrestrials have only been sent once, and thus any extraterrestrial civilizations receiving them would likely dismiss them.

So, have we ever sent a signal at least twice, to make sure that anyone receiving it would realize it is of artificial origin?

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  • $\begingroup$ Related: Does man send any signal to attract aliens? $\endgroup$ – TildalWave Jan 4 '14 at 20:40
  • $\begingroup$ @TildalWave Thanks, I should have linked that, it was what made me wonder. I knew about the Arecibo Message, but it was only broadcast once, so there was no way to prove it originated from an artificial source. $\endgroup$ – Canageek Jan 4 '14 at 21:45
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    $\begingroup$ Having a repetitive signal is good (though not perfect-think pulsars) evidence for artificial origin, but there are other ways. If we saw a signal that gave a million digits of $\pi$, we would probably be convinced it was artificial, even if we only saw it once. Similarly, our TV shows are probably repetitive enough to qualify, even if they are not identical. $\endgroup$ – Ross Millikan Jan 7 '14 at 23:47
  • $\begingroup$ @RossMillikan I set up another question about that: space.stackexchange.com/questions/3276/… $\endgroup$ – Canageek Jan 8 '14 at 6:46
  • $\begingroup$ Does almost seventy years of "I Love Lucy" count? $\endgroup$ – Rob Crawford Jan 11 at 22:06
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Yes, and one such list is compiled on the Wikipedia List of interstellar radio messages:

And I'm sure there were others we sent, wittingly or unwittingly. To just think how many times some TV programmes we watch were retransmitted, is mind boggling on its own. But that is beyond the point, because a whole message repetition isn't the only measure by which one can establish its artificial origins. Repetition is important, but it can be already present in a single signal session, inherent to message encoding and its contents.

For example, SETI's Lone Signal project is transmitting 144-character messages, sort of interstellar Twitter feed, towards Gliese 526. All these messages might be different when decoded as a whole, but their headers would repeat in some logical structure, following message contents. For example, you might want to send message length first, followed by the message itself, and then terminate all the messages equally. This should be sufficient to infer its artificial (but not necessarily intelligent) origins. ;)

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