there should definitely be an error, what should that error be?

I was thinking that fork should mean feed-death, so that no one can steal your identity. (when a pub sees two valid messages with the same sequence number) they could create a "fork proof" post, that links to both messages.

We'd have to refactor a bit around the database, currently it will simply not let you store invalid messages. Also, this will be necessary to allow retrival of one-off messages, which will be necessary for reposts.