Merge unless the other post was more than a certain time ago. I'd say about a day or something in that range. Also mark as unread, of course.

Stick with the merge, but leave a bold line running maybe 10 characters across, showing an update. If no posts are accumulated within X DD:HH:MM:SS, allow new post

And so the community has spoken. Merge it is.
I'll probably do an hour after the last post. Or maybe 2.