Can someone advice me how to prevent this error. An item with the same key has already been added.?

//  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);
        }

Error is here: sessionFactory = cfg.BuildSessionFactory();