Pro-forma comments are [a userscript][1]; anyone can implement another user script that does what you propose. Just as pro-forma comments are *not actually part of the official Stack Exchange interface*, a userscript could add this functionality.

###But this feature should not be implemented. At least not as described.

Questions aren't marked as duplicates by commenting. They are marked as duplicates by **flagging** or **voting to close as a duplicate**. Comments are secondary; they don't lead to closure, except insofar as someone may see the comment, decide they agree, and flag or vote.

Flagging or voting to close as a duplicate is not about *what duplicates are available*. It's about the question itself. Questions shouldn't be stretched to be duplicates of others. If you have to look through *a general master list* to know what question something is a duplicate of, you probably don't know it's a duplicate at all.

If you suspect a question is a duplicate and you go to flag or close it as one, you get a short list of similar-looking suggestions (this is separate from [the very useful, usually longer list Jorge Castro has described][2]). These suggestions indicate how many questions have been closed as duplicates of them:

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!["This question is a duplicate of which other question?" dialog][3]

The question listed first, with the most questions closed as duplicates, is not necessarily the best one. For example, this meta question is *not* a duplicate of the question where pro-forma userscript comments are shared.

But the same problem would happen if there were a short list of common dupe masters. That one would appear, and people would comment saying this was a duplicate of it.

The necessary thoughtfulness for selecting an appropriate duplicate would make a single short master list, giving the same selections for every possible duplicate, not appropriate in most circumstances.

Many duplicates are duplicates of the questions you listed. Most are not.


  [1]: http://stackapps.com/questions/2116/autoreviewcomments-pro-forma-comments-for-se
  [2]: http://meta.askubuntu.com/a/5899/22949
  [3]: https://i.sstatic.net/xYK40.png