The only
Cindy I've ever known was a
Lucinda. She was very pleasant and excellent company, but physically unremarkable bordering on plain: she had three exquisitely lovely sisters, and the family felt that a name as beautiful as
Lucy would have been inappropriate as a nn; hence the
Cindy!
I've been slack-jawed with amazement for decades.
Oh, and the only
Cynthia I know doesn't use a nn at all.