title: Fluent mapping base class - Tried to add property when already added
body: <p>I have a base class that looks like this:</p>
  
  <pre><code>public abstract class MyBaseClass
  {
      public virtual DateTime UpdatedOn { get; set; }
  }
  </code></pre>
  
  <p>I then have a series of other entities that inherit from this:</p>
  
  <pre><code>public class User : MyBaseClass
  {
      public virtual string UserName { get; set; }
      public virtual string Password { get; set; }
  }
  </code></pre>
  
  <p>My mapping for <code>User</code> would be:</p>
  
  <pre><code>public class UserMap : ClassMap&lt;User&gt;
  {
      public UserMap()
      {
          Id(x =&gt; x.UserName);
          Map(x =&gt; x.Password);
  
          Map(x =&gt; x.UpdatedOn);
      }
  }
  </code></pre>
  
  <p>I then have other entities, mapped the same way.<br>
  My problem is I get the following error:</p>
  
  <blockquote>
    <p>Tried to add property 'UpdatedOn' when already added.</p>
  </blockquote>
  
  <p>I guess this is because I map the <code>UpdatedOn</code> column in every entity?<br>
  Each of my tables has this UpdatedOn column, so how should I be mapping it?</p>