Annotation &1 should only be used once within a select list.

What causes this issue?

The following annotations should be used only once within the select or
element list of CDS objects:
@Semantics.currencyCode
@Semantics.unitOfMeasure
Elements having these annotations assigned are used as default
references for calculated elements of data type CURR/QUAN. In case of
multiple assignments it is not defined which of the elements is used as
reference.
In order to avoid this warning it is also possible to assign the
following annotations to all CURR-/QUAN-typed elements:
@Semantics.amount.currencyCode
@Semantics.quantity.unitOfMeasure

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.