exception handling oddity in third_party_auth

[deborahgu]

The error pasted below was flagged for Aperture's attention (because it bubbled up to user_api). The error is happening when user_api calls third_party_auth's get_duplicate_provider method and fails an assertion check because it apparently receives more than one "is already in use" Exception from social-auth-app-django (exception definition in python-social auth). Per both our comment and their docs, we should never get two messages here.

This only happened in one span, and maybe there was a weird glitch in the caller? I don't see any evidence the same user had a successful session, or a successful login since. (They have no recent log history, but they've enrolled in multiple courses in the past.)

error link (2U-internal).

Stack trace:

builtins.AssertionError
Traceback (most recent call last):
  File "/edx/app/edxapp/venvs/edxapp/lib/python3.11/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/contextlib.py", line 81, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/edx/app/edxapp/venvs/edxapp/lib/python3.11/site-packages/ddtrace/contrib/trace_utils.py", line 336, in wrapper
    return func(mod, pin, wrapped, instance, args, kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/edx/app/edxapp/venvs/edxapp/lib/python3.11/site-packages/ddtrace/contrib/internal/django/patch.py", line 333, in wrapped
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/edx/app/edxapp/venvs/edxapp/lib/python3.11/site-packages/django/contrib/auth/decorators.py", line 23, in _wrapper_view
    return view_func(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/edx/app/edxapp/venvs/edxapp/lib/python3.11/site-packages/django/views/decorators/http.py", line 43, in inner
    return func(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/edx/app/edxapp/edx-platform/openedx/core/djangoapps/user_api/accounts/settings_views.py", line 65, in account_settings
    duplicate_provider = pipeline.get_duplicate_provider(messages.get_messages(request))
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/edx/app/edxapp/edx-platform/common/djangoapps/third_party_auth/pipeline.py", line 439, in get_duplicate_provider
    assert len(social_auth_messages) == 1
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError