If you're client can't add their new credit card details and is receiving a "Credit Card with these details already exists", it's likely that they have the same credit card number but with a new expiry date.
We don't have access to the saved credit card number because of rules we have to comply with for handling credit card data.
However, you can update the card's expiry date by following these steps:
Go to your client's proposal > Invoices & Payments > Manage Payments > Select Arrange payment manually and Save.
Go back to Manage Payments and click on the 3 dots beside the CC and click delete
Afterwards, you or your client can now re-enter their CC details with the new expiry date.
Re-enable payments by going to the proposal's Invoices & Payments > Manage Payments > Select the new credit card > Save