On a rather big forum... there are always cases when a new thread pops up which is pretty similar to something that has already been posted and responded to in the past.
One solution is to close the thread after posting a link to the earlier thread.
The other solution is to merge the new thread with the older one.
I prefer to use the second one...
Users end up using the report button on vB to alert me about duplicate threads...
A system where users can suggest similar threads in the past for merging can come real handy to me and probably more forum administrators here.
Essentially, you can add a new section below named Similar Threads... And members can post relevant links there.
It would be visible to people in the thread. and moderators can be given a quick option to merge them all!