"Possible duplicate" warning after the first vote to close as a duplicate, is a new behavior. But it has some side effects:

• it does not appear as an action in the revisions list of a posting,
• the user who provided the link to the alleged duplicate is not identified
• the time at which the duplicate warning was added to the question, is not given

Previous methods of indicating a possible-duplicate question were visible and attributed to individual users.

This is the first feature that allows a hidden user action to alter the display of a question beyond ticking a counter (upvote, downvote, comment vote, page views).