There are 3 way a card can be attached to a customer profile. 

1) The customer profile is selected during checkout, and the customer pays with a card. 

2) Using the add card on file feature under the customer profile.

3) The customer can add a card by himself using the online booking website.

Please let me provide some more information about the first option. If tokenalization is enabled for store, the system will automatically save customers cards under the profile  that is selected during the sale. If no customer profile is selected during the charge, the card will not be saved. 

On our end, there is no way to validate if a card already saved under any profile since we save an encrypted credit card token, the card type (Visa, Mastercard, etc.), or the last four digits of the card. Since the last 4 digits and card type are not unique values, and we do not have access to information inside the card token, we cannot know if that card is already saved under another customer profile or not. 

There are two ways a customer profile can be attached to the card during a retail sale. 

1) The customer himself enters his phone number on the customer display. 

2) The customer was selected manually  by a front desk employee. 

In the case of booking a checkout, by the defalut customer is the same as the pet owner on the appointment, but it can be changed by the front desk employee.