Helps to deal with the case that reflection may report properties inherited from parent classes.
When we know about the fields already (inheritance has been anticipated in ClassMetadataFactory),
the driver must skip them.
The declaring classes may mismatch when there are private properties: The same property name may be
reported multiple times, but since it is private, it is in fact multiple (different) properties in
different classes. In that case, report the property as an individual field. (ClassMetadataFactory will
probably fail in that case, though.)