I recently added questions to the list of duplicates on a couple of closed questions. To my surprise I then found those questions at the top of the homepage. This was surprising to me because closing questions (as duplicates or for any other reason) does not bump them. After poking around a bit I found a couple of questions here that document that indeed the system is designed to not bump questions for being closed but to yes bump questions for having questions added to the duplicate list:
This seems rather puzzling. As far as I understand, the point of having modifications bump a question is so that the modifications can be reviewed. Actually closing a question seems to be a much more significant modification than merely adding an additional question to the duplicate list. As such I would expect there to be more of a reason for a question to be bumped when it gets closed than for for it to get bumped when the duplicate list is modified. Yet the opposite is apparently true.