A partner with partner function &1 already exists

What causes this issue?

Only one partner can be assigned to partner function &1 for each
notification.

System Response

The partner record is not created.

How to fix this error?

Assign only one partner to the notification.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.