I see that the system automatically adds "possible duplicate of..." links when voting to close as a duplicate.
Earlier today, I voted to close Why is SVN better than VSS? as a duplicate of SVN? VSS? Why is one better than the other?, and a suitable comment was added. I thought that comment would be valuable no matter what the close resolution ended up being. However, the question was ultimately closed as "subjective and argumentative", and the auto-added "possible duplicate" comment was removed!
I suggest that automatic "possible duplicate" comments only be removed if the question is actually closed as a duplicate. Otherwise, that useful information is lost.