[sqlsrvr]

For a forum and a topic, what are the natural keys for this? URL? (note that you can have subforums etc)

[r937]

natural key would be the name (and possibly the date, since forums/topics can theoretically have the same name)

[dportas]

In the NNTP protocol, it's the Message-ID. As far as I know all the other attributes of a post can be duplicated.

"Since the message-id field is unique with each article, it may be used by a news reading program to skip duplicate displays of articles that have been posted more than once, or to more than one newsgroup."
RFC 977: Network News Transfer Protocol

If you are building your own forum then whatever you like. It would seem like a good idea in principle to hash the content of the message and use a hash as the key. I suppose it depends on what the desired behaviour is when a post is duplicated. Maybe you actually want to give users the freedom to post the same message twice with the same timestamp, etc.