title: Issues with TPC inheritance with entity framework
body: <p>I have 2 objects that I want stored separately in 2 different tables. Let's call the classes Foo and FooTemp.</p>
  
  <p>I'm using Table Per Concrete Type (TPC) inheritance since I want class FooTemp to extend class Foo and I want the data to be stored separately.</p>
  
  <p>The only difference between the 2 tables is FooTemp has an extra field.</p>
  
  <p>I followed <a href="http://blogs.microsoft.co.il/blogs/gilf/archive/2010/01/25/table-per-concrete-type-inheritance-in-entity-framework.aspx" rel="nofollow">this guide for setting up TPC inheritance</a>. The only difference is that I have a extra field in my derived class.</p>
  
  <p>Now, here is my problem.
  When I add a new record to the database I get the error:</p>
  
  <blockquote>
    <p>"System.ArgumentException: An item
    with the same key has already been
    added"</p>
  </blockquote>
  
  <p>Why do I get this error? Is there any other way of doing inheritance and storing data in separate tables?</p>