Current line &1 already exists in line &2: duplicated key

What causes this issue?

While creating <GL:additional>additionals</>, you have entered data in
line &V1& that already appears as a <ZH>key</> in line &V2&.
No single combination of sales unit, <GL:class_type>class type</>,
<GL:class>class</> and additional may appear more than once in the
table. This combination forms the (primary) key, that must always be
unique.

How to fix this error?

Check your entries in the current line for the
Sales unit
Class type
Class
Additional
Note:
It is not possible to define more than one
<DS:GLOS.procedure_for_additionals>procedure for affixing an additional
</> per combination of additional, sales unit, class type and class.

Error message extract from SAP system. Copyright SAP SE.