Domain values may not be unique (see value table &1 in DDIC)

What causes this issue?

The current data object is bound to a data element in the ABAP
Dictionary (DDIC). The system has determined that the bound data element
is associated with value table &v1&. However, in this table, values may
exist that are formally unique only in combination with another key
field in the table. Since BRFplus cannot use value combinations (tuples)
instead of single values, there is a certain risk that the values
associated with the BRFplus data object may appear as duplicates or that
the expected value description texts are not displayed. . As a
consequence, it may be impossible for a user to decide to which table
entry a particular entry in the value list refers.
Note: This is not an error, but it can lead to unwanted effects and even
to errors in other BRFplus objects using the current data object.

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?

Try to set up a value table where the list of values that you want to
associate with the data object is unique in itself, that is, each value
may only occur once in the table.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.