No error will occur.
PayPay does not perform verification (duplicate checks) to determine if the referenceId provided during linking is already in use. We recommend implementing logic to detect and control duplicates on your system side.
Reason for recommending merchant-side checks
Since PayPay’s system specifications do not restrict duplicates, implementing the API without additional verification may allow a single PayPay user to link to multiple member accounts within your service.
Recommended action
To maintain the integrity, we recommend calling the PayPay User Authorization Status API (getUserAuthorizationStatus) before processing the link. Your system should then determine and control whether the target PayPay user is already associated with a different member ID.