I have noticed this and it makes sense not to have duplicates, but why not just merge any subsequent threads that are created into the original? It seems like it would help reduce the copious amounts of pages that some forum rooms have.

These type of threads are usually locked for a short while to alert the O.P as to their status and then deleted after a few days. Usually anyhow. If it is a thread that has had some replies then we will merge it.

In my experience most threads are closed after one post which is usually along the lines of 'hey what do you think of this band'.
You close it , give them a link to the thread already in existence and 9 times out of 10 they'll post something of more substance in that thread.