Currently, the OnTopic Library evaluates whether or not a key already exists for a given attribute within the same Topic.Parent and, if so, throws an exception. This is not enforced by the database itself, however, and can thus result in inadvertent duplicates when making manual updates to the Topics table, which will then throw an exception when loading the topics via e.g. an ITopicRepository implementation. This can be mitigated by adding a unique constraint on the TopicKey and ParentID columns.