lib/lp/bugs/doc/externalbugtracker-debbugs.txt fails intermittently in parallel tests

======================================================================
FAIL: lib/lp/bugs/doc/externalbugtracker-debbugs.txt
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python2.6/unittest.py", line 279, in run
    testMethod()
  File "/usr/lib/python2.6/doctest.py", line 2152, in runTest
    raise self.failureException(self.format_failure(new.getvalue()))
AssertionError: Failed doctest test for externalbugtracker-debbugs.txt
  File "lib/lp/bugs/doc/externalbugtracker-debbugs.txt", line 0

----------------------------------------------------------------------
File "lib/lp/bugs/doc/externalbugtracker-debbugs.txt", line 689, in externalbugtracker-debbugs.txt
Failed example:
    import_debian_bugs(['304014'])
Differences (ndiff with -expected +actual):
    - WARNING:...:Not importing debbugs #304014, since it's already
    - linked from LP bug(s) #1.

I could not dupe in an lxc in isolation. To investigate whether this can be duplicated by following the given test run, see the subunit output (including worker tags) in http://ubuntuone.com/2ToMVXFzwiYP2LUNG2CGbN .