The topic should reflect the content of the thread.
For off-topic discussion, a new thread should be created. (Occasional off-topic posts might be ok).
These rules already exist. They are part of "common sense". Use the report feature if you see this stuff.
Further discussion about existing topics should be posted in the existing thread(s).
I don't like this rule. Often there is a lot of new and useful discussion in "duplicate" threads. Even the constant "Deflationary=bad" threads get very good replies.