work around race condition in which two threads both try to create the same AccessToken

Author: n2ygk

oauth2_provider/oauth2_validators.py

@@ -335,7 +335,7 @@ def _get_token_from_authentication_server(
             try:
                 access_token = AccessToken.objects.select_related("application", "user").get(token=token)
             except AccessToken.DoesNotExist:
-                access_token = AccessToken.objects.create(
+                access_token, _created = AccessToken.objects.get_or_create(
                     token=token,
                     user=user,
                     application=None,