Should it be impossible for a session dict to return more than one value for a key?

[Michael]

In my project, I store the serial number of the device that logs in it's session. However in the code that gets the value from the session:

SESSION_SERIAL_NUMBER = 'terminal_serial_number'

user._serial_number = request.session.get(SESSION_SERIAL_NUMBER, '')

Very occasionally throws an error. The part that baffles me is:

get() returned more than one Session -- it returned more than 20!

How can a dict return more than one result, surely that is a bug for a dict to return more than one value?

Note: When I query the database django_sessions table with the given sessionid, it only returns one row.

Heres the error (from the email):

Internal Server Error: /accounts/login/

ProgrammingError at /accounts/login/
no results to fetch

And the traceback:

Traceback (most recent call last):
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/base.py", line 180, in _get_session
    return self._session_cache

During handling of the above exception ('SessionStore' object has no attribute '_session_cache'), another exception occurred:
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "./dist/terminals/middleware.py", line 11, in __call__
    user._serial_number = request.session.get(SESSION_SERIAL_NUMBER, '')
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/base.py", line 65, in get
    return self._session.get(key, default)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/base.py", line 185, in _get_session
    self._session_cache = self.load()
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/db.py", line 43, in load
    s = self._get_session_from_db()
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/db.py", line 32, in _get_session_from_db
    return self.model.objects.get(
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/query.py", line 443, in get
    raise self.model.MultipleObjectsReturned(

During handling of the above exception (get() returned more than one Session -- it returned more than 20!), another exception occurred:
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/base.py", line 180, in _get_session
    return self._session_cache

During handling of the above exception ('SessionStore' object has no attribute '_session_cache'), another exception occurred:
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/utils.py", line 97, in inner
    return func(*args, **kwargs)

The above exception (no results to fetch) was the direct cause of the following exception:
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/utils/deprecation.py", line 126, in __call__
    response = response or self.get_response(request)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/core/handlers/exception.py", line 49, in inner
    response = response_for_exception(request, exc)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/core/handlers/exception.py", line 110, in response_for_exception
    response = handle_uncaught_exception(request, get_resolver(get_urlconf()), sys.exc_info())
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/core/handlers/exception.py", line 149, in handle_uncaught_exception
    return callback(request)
  File "./core/base/views.py", line 152, in error_500_view
    response = render(request, "core.base/500.html", context=context)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/shortcuts.py", line 19, in render
    content = loader.render_to_string(template_name, context, request, using=using)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/template/loader.py", line 62, in render_to_string
    return template.render(context, request)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/template/backends/django.py", line 61, in render
    return self.template.render(context)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/template/base.py", line 174, in render
    with context.bind_template(self):
  File "/usr/lib/python3.8/contextlib.py", line 113, in __enter__
    return next(self.gen)
  File "/home/company/.venv/project/lib/python3.8/site-packages/debug_toolbar/panels/templates/panel.py", line 46, in _request_context_bind_template
    context = processor(self.request)
  File "./core/base/context_processors.py", line 13, in project
    menu = get_plug_menu(request.user)
  File "./dist/plug/menu.py", line 58, in get_plug_menu
    if user.is_anonymous or user.is_terminal:
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/utils/functional.py", line 248, in inner
    self._setup()
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/utils/functional.py", line 384, in _setup
    self._wrapped = self._setupfunc()
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/auth/middleware.py", line 25, in <lambda>
    request.user = SimpleLazyObject(lambda: get_user(request))
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/auth/middleware.py", line 11, in get_user
    request._cached_user = auth.get_user(request)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/auth/__init__.py", line 177, in get_user
    user_id = _get_user_session_key(request)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/auth/__init__.py", line 60, in _get_user_session_key
    return get_user_model()._meta.pk.to_python(request.session[SESSION_KEY])
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/base.py", line 50, in __getitem__
    return self._session[key]
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/base.py", line 185, in _get_session
    self._session_cache = self.load()
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/db.py", line 43, in load
    s = self._get_session_from_db()
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/contrib/sessions/backends/db.py", line 32, in _get_session_from_db
    return self.model.objects.get(
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/query.py", line 435, in get
    num = len(clone)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/query.py", line 262, in __len__
    self._fetch_all()
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/query.py", line 1354, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/query.py", line 51, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/sql/compiler.py", line 1234, in execute_sql
    return list(result)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/sql/compiler.py", line 1678, in cursor_iter
    for rows in iter((lambda: cursor.fetchmany(itersize)), sentinel):
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/models/sql/compiler.py", line 1678, in <lambda>
    for rows in iter((lambda: cursor.fetchmany(itersize)), sentinel):
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/utils.py", line 97, in inner
    return func(*args, **kwargs)
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/home/company/.venv/project/lib/python3.8/site-packages/django/db/utils.py", line 97, in inner
    return func(*args, **kwargs)

Exception Type: ProgrammingError at /accounts/login/
Exception Value: no results to fetch
Request information:
USER: AnonymousUser

This is the middleware it sits in:

from dist.terminals.apps import SESSION_SERIAL_NUMBER

class UserSerialNumberMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response
        # One-time configuration and initialization.

    def __call__(self, request):
        user = request.user
        user._serial_number = request.session.get(SESSION_SERIAL_NUMBER, '')
        response = self.get_response(request)
        return response