title: An item with the same key has already been added
body: <p>Can someone advice me how to prevent this error. An item with the same key has already been added.?</p>
  
  <pre><code>//  Failed to find a matching SessionFactory so make a new one.
          if (sessionFactory == null)
          {
              Check.Require(File.Exists(sessionFactoryConfigPath),
                            "The config file at '" + sessionFactoryConfigPath + "' could not be found");
  
              Configuration cfg = new Configuration();
              cfg.Configure(sessionFactoryConfigPath);
  
              /*MINE*/
              var persistenceModel = new PersistenceModel();
              persistenceModel.AddMappingsFromAssembly(Assembly.Load("EMedicine.Core"));
              persistenceModel.Configure(cfg);
              /*END_OF_MINE*/
  
              //  Now that we have our Configuration object, create a new SessionFactory
              sessionFactory = cfg.BuildSessionFactory();
  
              if (sessionFactory == null)
              {
                  throw new InvalidOperationException("cfg.BuildSessionFactory() returned null.");
              }
  
              if (sessionFactoryConfigPath != null) sessionFactories.Add(sessionFactoryConfigPath, sessionFactory);
          }
  </code></pre>
  
  <p>Error is here:
  sessionFactory = cfg.BuildSessionFactory();</p>