Maybe when people start up a new thread that's on the same subject as an existing one the etiquette ought to be that they explain why. That happens sometimes, but it get's left out too often.

There often is a perfectly good reason - there is some aspect of the topic which would otherwise get in the way of a discussion, but which deserves exploring. And if there's a good reason it deserves to be explained. (It gets confusing when several discussions are being carried on in the same thread - for example one about electoral procedures,one about policies, one about media coverage, one about the previous election...)
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Here's a collection of treads.