In this case, one thread was created from a news article posted on the front page and the other by a user directly posting a link to the article on the original article in question. This clogs up the forums and disjoints actual discussion.
This duplication of threads can also occur when there is a long-running, existing thread on a topic (such as Inland Rail) and people creating new threads as further news articles on the topic occur do not bother to search for existing topics. In this case, the merger of the 'news article discussion' thread with the existing topic would provide extra context to those unfamiliar with the topic at hand.
There are several possible ways to remedy this issue:
An 'existing thread merger' flag for reports so that moderators can merge the threads if necessary. The description of the flag would ask the reporter to add a link to the existing thread.
An 'existing thread' button similar to the herring button that would open a dialogue box to ask the user to post a link to an existing topic
Enable other users to point a news article to an existing discussion thread. Currently only the original article poster can do this, but it is often ignored.