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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?
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. ;)
