duplicatemappingexception

NHibernate DuplicateMappingException when mapping abstract class and subclass

I have an abstract class, and subclasses of this, and I want to map this to my database using NHibernate. I'm using Fluent, and read on the wiki how to do the mapping. But when I add the mapping of the subclass an NHibernate.DuplicateMappingException is thrown when it is mapping. Why? Here are my (simplified) classes: public abstract...