On startup the following exception sometimes occur. The application has multiple threads that use NHibernate separately against different databases.

System.ArgumentException: An item with the same key has already been added.
at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
at NHibernate.Util.ThreadSafeDictionary`2.Add(TKey key, TValue value)
at NHibernate.Type.TypeFactory.AddToTypeOfNameWithLength(String key, IType type)
at NHibernate.Type.TypeFactory.GetType(NullableType defaultUnqualifiedType, Int32 length, GetNullableTypeWithLength ctorDelegate)
at NHibernate.Type.TypeFactory.<RegisterBuiltInTypes>b__1c(Int32 l)
at NHibernate.Type.TypeFactory.BuiltInType(String typeName, Int32 length)
at NHibernate.Mapping.SimpleValue.GetHeuristicType()
at NHibernate.Mapping.SimpleValue.get_Type()
at NHibernate.Mapping.SimpleValue.IsValid(IMapping mapping)
at NHibernate.Mapping.Property.IsValid(IMapping mapping)
at NHibernate.Mapping.PersistentClass.Validate(IMapping mapping)
at NHibernate.Mapping.RootClass.Validate(IMapping mapping)
at NHibernate.Cfg.Configuration.ValidateEntities()
at NHibernate.Cfg.Configuration.Validate()
at NHibernate.Cfg.Configuration.BuildSessionFactory()