Multiple main GTINs selected for material &1 and unit of measure &2

What causes this issue?

Multiple GTINs were marked as the main GTIN for article &V1& and unit of
measure &V2&; this is not allowed.

System Response

The affected data records for article &V1& and unit of measure &V2& were
marked as having errors.

How to fix this error?

Adjust the <ZK>Main GTIN</> indicator so that exactly one GTIN is
classified as the main GTIN for each article and unit of measure.
You can correct the error in two ways:
By adjusting the data in your input file/in the clipboard
In the transaction, by correcting the <ZK>Main GTIN</> indicator in one
of the incorrect GTINs for this article and unit of measure.

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.