fix issue where trying another already confirmed email threw an uncaught exception (#11161)

## Purpose
If user email already existed the error was uncaught.


## Changes
- adds change to catch 400 error 
- adds test case.

Author: Johnetordoff

api/users/views.py

@@ -2,6 +2,7 @@
 from urllib.parse import urlencode
 
 from django.apps import apps
+from django.db import IntegrityError
 from django.db.models import F
 from django.utils.decorators import method_decorator
 from django.views.decorators.csrf import csrf_protect
@@ -1146,7 +1147,12 @@ def post(self, request, *args, **kwargs):
         if not user.is_registered:
             user.register(email)
 
-        user.emails.get_or_create(address=email.lower())
+        if not user.emails.filter(address=email.lower()).exists():
+            try:
+                user.emails.create(address=email.lower())
+            except IntegrityError:
+                raise ValidationError('Email address already exists.')
+
         user.date_last_logged_in = timezone.now()
 
         del user.email_verifications[token]

api_tests/users/views/test_user_confirm.py

@@ -199,3 +199,31 @@ def test_post_success_link_with_email_verification_none(
 
         user.reload()
         assert not user.external_identity
+
+    @mock.patch('website.mails.send_mail')
+    def test_post_success_link_with_email_already_exists(
+            self,
+            mock_send_mail,
+            app,
+            confirm_url,
+            user_with_email_verification
+    ):
+        user, token, email = user_with_email_verification
+        AuthUserFactory(username=email)  # User with already existing email
+        user.save()
+
+        res = app.post_json_api(
+            confirm_url,
+            {
+                'data': {
+                    'attributes': {
+                        'uid': user._id,
+                        'token': token,
+                        'destination': 'doesnotmatter'
+                    }
+                }
+            },
+            expect_errors=True
+        )
+        assert res.status_code == 400
+        assert res.json['errors'][0] == {'detail': 'Email address already exists.'}