MultipleObjectsReturned at /admin/events/registration/add/ get() returned more than one Member -- it returned 2!

One-sentence description

MultipleObjectsReturned at /admin/events/registration/add/ get() returned more than one Member -- it returned 2!

Current behaviour

MultipleObjectsReturned at /admin/events/registration/add/
get() returned more than one Member -- it returned 2!

Request Method: POST
Request URL: https://thalia.nu/admin/events/registration/add/?_changelist_filters=p%3D43
Django Version: 1.11.4
Python Executable: /usr/local/bin/uwsgi
Python Version: 3.5.4
Python Path: ['.', '', '/usr/local/lib/python35.zip', '/usr/local/lib/python3.5', '/usr/local/lib/python3.5/plat-linux', '/usr/local/lib/python3.5/lib-dynload', '/usr/local/lib/python3.5/site-packages']
Server time: Thu, 7 Sep 2017 18:38:06 +0200
Installed Applications:
['django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.sites',
 'django.contrib.sitemaps',
 'tinymce',
 'rest_framework',
 'rest_framework.authtoken',
 'compressor',
 'corsheaders',
 'thaliawebsite',
 'members',
 'documents',
 'activemembers',
 'photos',
 'utils',
 'mailinglists',
 'merchandise',
 'thabloid',
 'partners',
 'events',
 'pizzas',
 'newsletters',
 'education',
 'announcements',
 'django_slack']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'corsheaders.middleware.CorsMiddleware',
 'django.middleware.http.ConditionalGetMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware',
 'django.middleware.locale.LocaleMiddleware']


Traceback:  

File "/usr/local/lib/python3.5/site-packages/django/core/handlers/exception.py" in inner
  41.             response = get_response(request)

File "/usr/local/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
  187.                 response = self.process_exception_by_middleware(e, request)

File "/usr/local/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
  185.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/usr/local/lib/python3.5/site-packages/django/contrib/admin/options.py" in wrapper
  551.                 return self.admin_site.admin_view(view)(*args, **kwargs)

File "/usr/local/lib/python3.5/site-packages/django/utils/decorators.py" in _wrapped_view
  149.                     response = view_func(request, *args, **kwargs)

File "/usr/local/lib/python3.5/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
  57.         response = view_func(request, *args, **kwargs)

File "/usr/local/lib/python3.5/site-packages/django/contrib/admin/sites.py" in inner
  224.             return view(request, *args, **kwargs)

File "/usr/local/lib/python3.5/site-packages/django/contrib/admin/options.py" in add_view
  1508.         return self.changeform_view(request, None, form_url, extra_context)

File "/usr/local/lib/python3.5/site-packages/django/utils/decorators.py" in _wrapper
  67.             return bound_func(*args, **kwargs)

File "/usr/local/lib/python3.5/site-packages/django/utils/decorators.py" in _wrapped_view
  149.                     response = view_func(request, *args, **kwargs)

File "/usr/local/lib/python3.5/site-packages/django/utils/decorators.py" in bound_func
  63.                 return func.__get__(self, type(self))(*args2, **kwargs2)

File "/usr/local/lib/python3.5/site-packages/django/contrib/admin/options.py" in changeform_view
  1408.             return self._changeform_view(request, object_id, form_url, extra_context)

File "/usr/local/lib/python3.5/site-packages/django/contrib/admin/options.py" in _changeform_view
  1440.             if form.is_valid():

File "/usr/local/lib/python3.5/site-packages/django/forms/forms.py" in is_valid
  183.         return self.is_bound and not self.errors

File "/usr/local/lib/python3.5/site-packages/django/forms/forms.py" in errors
  175.             self.full_clean()

File "/usr/local/lib/python3.5/site-packages/django/forms/forms.py" in full_clean
  384.         self._clean_fields()

File "/usr/local/lib/python3.5/site-packages/django/forms/forms.py" in _clean_fields
  402.                     value = field.clean(value)

File "/usr/local/lib/python3.5/site-packages/django/forms/fields.py" in clean
  160.         value = self.to_python(value)

File "/usr/local/lib/python3.5/site-packages/django/forms/models.py" in to_python
  1238.             value = self.queryset.get(**{key: value})

File "/usr/local/lib/python3.5/site-packages/django/db/models/query.py" in get
  384.             (self.model._meta.object_name, num)

Exception Type: MultipleObjectsReturned at /admin/events/registration/add/
Exception Value: get() returned more than one Member -- it returned 2!
Request information:
USER: mvstiphout

GET:
_changelist_filters = 'p=43'

POST:
event = '212'
date_cancelled_0 = ''
name = ''
date_1 = '18:37:49'
date_cancelled_1 = ''
initial-date_0 = '2017-09-07'
csrfmiddlewaretoken = 'x'
initial-date_1 = '18:37:49'
member = '1219'
_save = 'Save'
date_0 = '2017-09-07'

Expected behaviour

No error

Steps to reproduce

1. Register Justin for an event