If a bug report is marked as a duplicate, Launchpad (almost completely) prevents people from changing the report's context(s), status(es), importance(s), or milestone(s) because there is no longer any point in changing those things, so letting people do so would be misleading.
However, Launchpad also prevents people from changing the bug report's assignees, and this causes unwanted bugmail if chatter continues in the bug report.
[This problem originally reported by Daniel Holbach in bug 54801.]
Ways we could fix this:
1. Prohibit comments in duplicate bug reports. But this would mean that discussions about whether a report really was a duplicate would happen in the original instead, which would both inconvenience subscribers to the duplicate, and annoy subscribers to the original.
2. Not send out any bugmail for duplicate bug reports. But this would make any legitimate discussion about whether the report was really a duplicate much less effective.
3. Let assignees to duplicates unassign themselves.
Well, as for point #1, it already annoys subscribers to the original, who get broadcast messages in any of the duplicates. I agree with your consideration for #2, though I think incorrect dupes are actually not that frequent to warrant such special handling.
You didn't propose a #4, which is unassigning and unmilestoning when duplicating (adding a comment if you would like to store the history of who the assignee was), and then hiding the whole table. I underline my strong opinion that the assignee of a dupe is not very important at all.