EAN already used for unit of measure & of this material

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

What causes this issue?

You have entered an EAN that is already being used for another unit of
this material.

System Response

How to fix this error?

Check to see whether you really want to assign this number, or correct
your entry.

Error message extract from SAP system. Copyright SAP SE.