Uploading an event with GPS time identical to an existing event always triggers a "Request would not modify the superevent" error

To reproduce, just download an existing event and upload it again:

gracedb -s https://gracedb-playground.ligo.org/api/ download M1432 coinc.xml
gracedb -s https://gracedb-playground.ligo.org/api/ CBC gstlal MDC coinc.xml

In the log, you will see:

[2018-10-26 07:55:28,871: ERROR/ForkPoolWorker-1] Task gwcelery.tasks.superevents.handle[e6345b47-3e36-4d46-a85b-807fe364ab7e] raised unexpected: HTTPError(400, 'Bad Request', b'"Request would not modify the superevent"')
Traceback (most recent call last):
  File "/home/emfollow/.local/lib/python3.6/site-packages/celery/app/trace.py", line 382, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/celery/app/trace.py", line 641, in __protected_call__
    return self.run(*args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/gwcelery/tasks/superevents.py", line 122, in handle
    t_end=new_t_end)
  File "/home/emfollow/.local/lib/python3.6/site-packages/gwcelery/tasks/superevents.py", line 240, in _update_superevent
    gracedb.update_superevent(superevent_id, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/celery/app/trace.py", line 642, in __protected_call__
    return orig(self, *args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/celery/app/task.py", line 375, in __call__
    return self.run(*args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/celery/app/base.py", line 467, in run
    return task._orig_run(*args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/gwcelery/tasks/gracedb.py", line 152, in update_superevent
    t_0=t_0, preferred_event=preferred_event)
  File "/home/emfollow/.local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 833, in updateSuperevent
    return self.patch(uri, body=request_body)
  File "/home/emfollow/.local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 298, in patch
    return self.post_or_put_or_patch("PATCH", *args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 321, in post_or_put_or_patch
    return self.request(method, url, body, headers)
  File "/home/emfollow/.local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 521, in request
    return GsiRest.request(self, method, *args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 262, in request
    return self.adjustResponse(response)
  File "/home/emfollow/.local/lib/python3.6/site-packages/ligo/gracedb/rest.py", line 275, in adjustResponse
    raise HTTPError(response.status, response.reason, response_content)
ligo.gracedb.exceptions.HTTPError: (400, 'Bad Request', b'"Request would not modify the superevent"')