Observe this link : https://developer.rhino3d.com/api/RhinoCommon/html/T_Rhino_Geometry_AnnotationBase.htm#fullInheritance
and this : https://developer.rhino3d.com/api/RhinoCommon/html/T_Rhino_Geometry_Dimension.htm
RadialDimension are duplicates.
If you inspect these classes with your IDE, you can understand that these classes are subclasses of the
but if you read the API page, these classes are defined as subclasses of the AnnotationBase class.
(for example: https://developer.rhino3d.com/api/RhinoCommon/html/T_Rhino_Geometry_AngularDimension.htm)