PO item & included in IDoc twice

What causes this issue?

Purchase order item &V1& occurs in several IDOC item segments
simultaneously.
The units of measure or the action codes in these IDOC item segments do
not agree.

How to fix this error?

Get the supplier to ensure that IDOC item segments relating to the same
PO item in these fields agree, or change the IDOC so that these fields
agree with the PO data, and then re-start the process.

Error message extract from SAP system. Copyright SAP SE.