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Duplicate abstract operation (Reviewer Review)
Checks whether classes have abstract operations that are already declared on superclasses.
This often happens when an operation is control+dragged to copy it to a subclass.
Consequences
This is not a valid model and cannot be implemented, but the problem is minor.
Strategy
Typically, you will want to clear the abstract checkbox from the subclass.
When refining operations using the class browser the abstract option will be automatically removed, so this can be a good practice.