Marking bug report as duplicate of its duplicate using the API causes endless loop, DB timeouts

Using the web UI (which uses the LP.API via the form overlay):

1. Open a bug report that has a duplicate.
2. Click "Mark as duplicate".
3. Type the number of the bug report that is the duplicate of this one.
4. Press Enter.

What happens:
4. The duplicate form disappears.
5. A small spinner appears next to the "Mark as duplicate" link for several seconds.
6. The dialog reopens, displaying a massive amount of SQL gibberish in red.

What should happen:
3. Instantly upon entering a number equal to that of a bug report that was a duplicate of this one when this page was loaded, the Confirm button should be disabled and Launchpad should show an error caption under the bug number field explaining why you can't do that. (The page already lists which bug reports are duplicates of this one, so this would not require any extra effort from the database.)
4. If there's a possibility that it will need to be visible in a few seconds, the duplicate form should not disappear in the first place.
6. If the pre-emptive error prevention didn't work (for example, if someone else marked Y as a duplicate of X while you had X open in preparation for marking it as a duplicate of Y), the error should be in English rather than SQL.

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Using launchpadlib:

{{{
bug = lp.bugs[147229]
bug.duplicate_of = bug
bug.lp_save()
}}}

results in an endless loop followed by a DB timeout.