When two issues are linked, it would be useful if some fields on the issue were propagated to the linked issue.

For example, imagine you have a user who reports an issue and is watching his email notifications to see how things are going progressing with the bug (and perhaps there are a few other users who are watching the bug as well). If that issue is then marked as a duplicate, it would be nice if all of those users who were getting notifications for the original bug, started receiving notifications for the new issue as, from their point of view, it is the same issue and they are still interested in it, it just happens to have a different issue ID.