False Ciri was a girl who resembled Ciri and was abducted by mistake because of this. This 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.