Normally, I don't see an edit in the revision history where the possible duplicate banner is added after someone votes to close a question but take a look at this post, not only can I view the banner being added but I can also edit it out of the post.
In the past, Community would actually edit that message into a post when it was closed as a duplicate, as can be seen in Revision 2 of that post.
This behaviour was changed as part of last year's large overhaul of the closing system.
A long time ago, the duplicate banner was part of the question itself. It used to get automatically added by the Community user the moment the question is closed as such.
Not anymore though. It's now a separate entity that normal users can't touch. (at least not without reopening the question)
As you can see, Community actually added in the duplicate link in the second edit. Community used to do this, now it is not part of the source, but an external force that we can't do anything about unless we open the question