This issue can present itself when a customer profile already exists within Authorize.net while using the Authorize.net API to create a new profile.
There are several issues here and there currently isn't a solution as it is a limitation by the Authorize.net API:
Authorize.Net will not return search results if there are multiple customers with the same email address in the Auth.Net customer portal
If we can't retrieve the customer from Auth.Net we can't link them
Even if we could find the Auth.Net account, we cannot link a new customer to an existing credit card just because they entered an existing email address (major security concern)
How to fix:
The only way to fix this currently is to go into Authorize.Net directly, delete the customer profile, and have the customer try again
References:
Search by Customer Profile - https://developer.authorize.net/api/reference/index.html#customer-profiles-get-customer-profile
Duplicate Verification - https://developer.authorize.net/api/reference/features/customer_profiles.html#Duplicate_Profile_Verification