When a public bug is marked as a duplicate of a private bug, the UI for those who can only see the public bug is confusing. In the public bug, the "Duplicate of bug #NNNNN" text in the upper right (beneath "This report is public") gives the number of the private bug, but is *not* a link.
For the user, this is frustrating. They can see that the public report is a duplicate of something private, but they have no way to see the private thing (fair enough) and no way to ping the owner(s) of the private thing to ask them if it really needs to be private. Maybe it does: if it's an apport filing, for example, it may contain private crash data in the attachment, even though the bug itself is not a security vulnerability.
Apport deliberately does this - see bug 764414.
One solution might be to distinguish between:
1) Bugs that are inherently private, because (say) they are security vulnerabilities.
2) Bugs where the bug itself can be public, but the reproduction data is private.
In case (2), the bug could be public and just the attachment be kept private. As long as the attachment clearly explained why it wasn't public, this would be a vast improvement, both in terms of making more bug information available and in not frustrating users.
Meanwhile, when a public bug is marked as a dup of a (1)-style private bug, then at least the "Duplicate of bug #NNNNN" could be a link to some page that offers some kind of explanation for why you can't see the real bug. That would obviate the need to keep making answers like these:
* bug #334130 ("misbehaviour in marking duplicates of a private bug")
* bug #157899 ("Shouldn't need access to private bug report to reverse a public duplicate marking")
Solving this bug would probably solve most of #334130 (this might even be considered a dup of that one, but it's hard to say, as 334130 is partly about the UI for actually marking the dup, whereas this is strictly about the UI for encountering an already-marked dup).