Fluent duplicate mapping problem

FluentMapping.cs

 public class ArticleListMap : SubclassMap<Article>
  {
      ArticleListMap()
      {
          DiscriminatorValue(1);
      }
  }
public class ArticleMap : SubclassMap<Article>
  {
      ArticleMap()
      {
          DiscriminatorValue(4);
          Map(x => x.ExtraText);
          HasManyToMany(x => x.TagSet);
          HasManyToMany(x => x.ImageSet);
      }
  }