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.