bDuplicatingReadOnly

Flag indicating that a read only duplicate of this blueprint is being created, used to disable logic in ::PostDuplicate,

This flag needs to be copied on duplication (because it's the duplicated object that we're disabling on PostDuplicate), but we don't need to serialize it for permanent objects.

Without setting this flag a blueprint will be marked dirty when it is duplicated and if saved while in this dirty state you will not be able to open the blueprint. More specifically, UClass::Rename (called by DestroyGeneratedClass) sets a dirty flag on the package. Once saved the package will fail to open because some unnamed objects are present in the pacakge.

This flag can be used to avoid the package being marked as dirty in the first place. Ideally PostDuplicateObject would not rename classes that are still in use by the original object.