Comment 9 for bug 46237

Whether knowing that a bug was marked as a duplicate of a bug you're subscribed to is interesting is determined by the amount of information in the duplicate, and to some extent its age.

If a an old bug with many comments is marked as a duplicate then the subscribers of the target bug might want to know about it so that they can have a look and see if there's any helpful information. For bugs that are filed and soon after marked as duplicates, this is indeed annoying.

I'm not sure I'm against canceling those emails altogether, but if we wanted to preserve some of their utility we could conditionally only send an email if the duplicate has comments, and is older than one day.