Duplicate profile error

What are the conditions in which this error appears on the checkout page:

“A duplicate customer payment profile already exists.”

A customer is trying to make a one-time transaction again. The first time she tried, her card was declined from her bank. She sorted it out and they said she’s good to go. So she tried again today. She logged in to her existing account and tried to make a new payment and got that error message. Why is this error appearing?