"Do you really wish to know?" Spoilers from the books and/or games to follow!
I've found a certain girl. She's from the Cintra nobility, a war orphan. She's been through refugee camps, and is currently measuring cloth in ells and cutting it out, having been taken in by a Brugge draper. There is nothing remarkable about her, aside from one thing. She is quite similar in likeness to a certain miniature of the Lion Cub of Cintra ... Fancy a look?
False Ciri was a girl of roughly the same age and somewhat similar appearance as Ciri. Because of this, she was abducted and taken back to Nilfgaard.
"Have you not understood? Instead of the real Cirilla of Cintra I've been sent some kind of dolt. Those traitors probably told themselves that I would not recognise her. But I will know the real Ciri. I would know her at the end of the world and in the darkness of hell."
The emperor knew immediately that she was not Ciri, but decided that the people of Nilfgaard should remain none the wiser. He sent her off to the farthest reaches of the empire while he continued his search for the real Ciri.
Later accounts of this tale suggest that the incident happened during the emperor's later period, which was not known to be his most lucid. Emhyr married her and she became the empress Cirilla, and to all intents and purposes the Nilfgaardian people were none the wiser and accepted her as Ciri.
In the end, Emhyr grew to love her anyway, so she remained.
Illustration is by Jana Komarková and comes from czech issue of Lady of the Lake.
Though she never features in the release version of The Witcher 3: Wild Hunt, "False Ciri" was mentioned in leaked materials from 2014. She was to appear on the Skellige Isles, and use another name - "Becca" (the same as the first Queen of Cintra). It was not explained whether or not this was her real name or just another alias.