Short comments are sometimes better.
Although I'm somewhat infamous for posting long comments myself, I believe there is often a benefit to a comment that is short:
- It can be read quickly.
- It doesn't take up much space. One reason that is good is it means it doesn't impinge much on the visibility of other comments.
- It doesn't appear overly authoritative. A "possible duplicate" comment does not reflect even a weak form of community consensus; it's just a suggestion.
Sometimes a longer comment is better than a shorter comment. I think this is especially likely to happen when:
- The longer comment expresses something especially complicated and it's long because a shorter version would be confusing or misleading. Or:
- The longer comment is written in such a way to address the specific needs of the question or answer it's posted on. Its length reflects the time and thought a person put in to the specific question at hand.
In contrast, I think shorter comments are especially likely to be better than longer comments when:
- The greater length of the longer comment would owe mainly to meta-information, such as information on how to use the site.
- The comment is reusable and the fundamental idea it expresses is not complicated or particularly confusing or non-intuitive.
- The comment is being posted by an automated or semi-automated process.
- The additional information in a longer comment can equally or more effectively be provided by linkifying (hyperlinking) some of the text in the shorter comment to a page that offers a full explanation. Unlike answers, comments are not expected to be self-contained.
These comments are automatic... but that's not as important as it seems.
As Braiam says, most "possible duplicate" are generated automatically. This usually happens when a user votes to close a question as a duplicate of another question. Roughly speaking, the way the system decides whether or not to post a "possible duplicate" comment in a user's name is:
- If there's already a comment linking to the proposed duplicate master, no automatic comment is posted.
- If there is no comment the system recognizes as pointing to the proposed duplicate, an automatic comment is posted. If the only comment(s) linking to the master question link only to answers on it, an automatic comment with a link to the question (i.e., the top of the question page) is still posted.
(But there may be bugs or quirks that produce other behavior under some circumstances.)
Although most "possible duplicate" comments are posted automatically by the system, they are still the responsibility of the user in whose name they are posted.
- It is intentional that the system posts them in the name of that user. If the user is pinged in a reply (e.g.,
@EliahKagan in a comment coming after a possible duplicate comment posted in my name), they are notified.
- The user in whose name the comment is posted "owns" the comment: they may delete it, and (within five minutes of when it is posted, just like any other comment) they may edit it.
If it would be inappropriate to post a "possible duplicate" comment manually, a user should edit or delete/replace their "possible duplicate" comment with something else.
But I don't think a simple "possible duplicate of [link]" comment is usually inappropriate; short comments seem best for this situation. Also, I don't think a short comment--even one that's not quite a complete sentence--is rude.
Could "possible duplicate" comments be more instructive and informative?
Part of your concern seems to be that newcomers may not know how we handle duplicate questions here on Ask Ubuntu, or may not know what to say or do when the proposed master question doesn't actually help. They might not have to do anything though--sometimes when someone votes to close a question as a duplicate of another, it really isn't (and usually doesn't get closed).
But I can think of one way to keep the stock "possible duplicate" comment the same length, while providing more information to people who don't quite know how to use the site yet, would be to linkify the word duplicate to point to a help page explaining how duplicates work. This help page might be appropriate, though I'd be concerned it might deceive people into thinking their question had already been closed as a duplicate.
If automatic comments should be short, why are some "review comments" long?
"Possible duplicate" comments are sometimes compared to the comments people have the option to select after clicking Recommend Deletion (or Delete) in the low quality posts review queue (or to the similar pro-forma comments).
But when the system posts a "possible duplicate" comment in a user's name, that is automatic in a different way than a review comment is "automatic." A "possible duplicate" comment is posted when the user does an action not fundamentally related to commenting: voting to close a question as a duplicate.
In contrast, a review comment is posted when the user chooses to select one instead of keeping the default option of not posting a comment. Review comments (and pro-forma comments) are shown to a user in full before they post it; they are not a side effect of some other action. Review and pro-forma comments are only automatic in the sense that the user doesn't have to manually type out the text of the comment. We're expected to give those "automatic" comments the same deliberate consideration we'd apply to a comment we typed our ourselves.
As a separate distinction, some review and pro-forma comments express complex ideas or are based on an assumption that the author of a post isn't aware of how to properly use the site (a reasonable assumption for a user who posts a "thanks" or "me too" answer).
In contrast, commenting to say someone's question may be a duplicate of another question is quite intuitive--question authors probably know that this is a suggestion to take a look at the other question and see if it helps, even without any special explanation.