Assignment to PO item is not unique

What causes this issue?

The selection using criteria delivery note &V1& and supplier &V2& is
ambiguous. There are two possible causes for this error:
More than one delivery note &V1& exists for supplier &V2& OR
The selected purchase order contains the material in more than one item.

How to fix this error?

The PO number and PO item number cannot be determined uniquely. For this
reason, the system must transfer this data to the IDoc in segment
E1EDP02 / Qualifier 002.
If you are using a SOA message, you must transfer the PO number in the
PurchaseOrderReference-ID field, and the PO item number in the
PurchaseOrderReference-ItemID field.

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.