We have to start with a huge disclaimer: there are two Cunégondes.
No, Cunégonde doesn't have a doppelganger. She's not an evil twin. She doesn't have split personalities. But there's Cunégonde herself, in all of her 100% Cunégonde-itude, and then there's the Cunégonde of Candide’s imagination... who (no shocker here) is, like Mary Poppins: practically perfect in every way.
Like the Old Woman, Cunégonde (the real woman) is subject to horrific violence, both sexual and otherwise:
"I was in bed and fast asleep when it pleased God to send the Bulgarians to our delightful castle of Thunder-ten-Tronckh; they slew my father and brother, and cut my mother in pieces. A tall Bulgarian, six feet high, perceiving that I had fainted away at this sight, began to ravish me; this made me recover; I regained my senses, I cried, I struggled, I bit, I scratched, I wanted to tear out the tall Bulgarian's eyes—not knowing that what happened at my father's house was the usual practice of war. The brute gave me a cut in the left side with his hanger, and the mark is still upon me." (8.1)
Initially a woman of high social standing, she is reduced to servitude. Ultimately, her horrific experiences destroy her beauty and temperament. Cunégonde seems to represent the way the horrors of the world destroy innocence and beauty.
As Candide perceives her, Cunégonde is a woman of endless virtue, innocence, contentment, and beauty. When Candide and Cunégonde are finally reunited at the end of the novel, the real Cunégonde (who is, surprise, surprise, unable to live up to Candide’s expectations) comes as an inherent disappointment to Candide:
At the bottom of his heart Candide had no wish to marry Cunégonde. But the extreme impertinence of the Baron determined him to conclude the match, and Cunégonde pressed him so strongly that he could not go from his word. (30.1)
In much the same way, the world has proved a disappointment to his once endless Optimism.