if you have nested classes that need to be overriden you normally will use the keyword new to hide base properties. But if you do that the serializer complains about duplicate property names. The message it gives is correct but in the supplied case this is not a problem but the intention todo so. Sample and details are in stackoverflow

https://stackoverflow.com/questions/55069966/bson-does-not-serialize-with-new-overridden-attributes-and-reports-duplicate-u