I don't know if anyone else thinks this is a good idea, so post your impressions. If you don't like my idea then I will promptly hit abuse, however. ;^)
Sure is - that is one of our jobs already. Often we don't see the new (or later higher) thread until after it has been there for a time. But rest assured, I remove older ones as soon as I see the new ones. Trouble is, I only look every 10 to 15 minutes - sometimes more. Got too many other things going.
Sometimes there are fewer moderators on than at other times, that also contributes to older multi-thread posts not being seen and removed. I would like to see the poster have the ability to remove his own posts from the sidebars if he sees the need to do so. That may come one day.