Bug: import_alums fails when there is more than one user with the same email address

[pdpinch]

I'm not sure how this happens in production, but we have multiple user accounts with the same email address.

In these cases, the import_alums management command fails with this traceback:

Traceback (most recent call last):
  File "/app/profiles/api.py", line 397, in import_alumni
    import_alum(alum)
  File "/app/profiles/api.py", line 299, in import_alum
    user = User.objects.get(email__iexact=alum.learner_email)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/app/.heroku/python/lib/python3.6/site-packages/django/db/models/query.py", line 412, in get
    (self.model._meta.object_name, num)
django.contrib.auth.models.MultipleObjectsReturned: get() returned more than one User -- it returned 3!

When there is more than once account with the same email, we could chose the one that has been logged in to most recently.

[mbertrand]

Looks like there are lots of users with dupe email accounts on prod (536 dupe emails), in the worst case 6 Users with the same email (but only 1 of those ever logged in).

This complicates hubspot api syncing, and specifically tracking the hubspot id associated with each contact.

Hubspot only allows 1 contact per email, so users with dupe emails would all be associated with the same hubspot id, but that will lead to an integrity error in our database (only 1 contact per hubspot id allowed).

Not sure if I should remove that integrity constraint or fix the underlying issue causing dupe emails and remove the users that never logged in. Of the 536 dupes, there are only 4 where >1 User logged in.