In &1, the &3 is allocated multiple times

What causes this issue?

The payment advice note contains multiple items for the external
document number.
Check whether the item appears twice in the payment advice note, but has
only been paid once.

System Response

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

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.