Fix for GWE-282-0 (#5126)

* addressing GWE-282-0

* fix isort

app/marketing/views.py

@@ -25,6 +25,7 @@
 from django.conf import settings
 from django.contrib import messages
 from django.contrib.admin.views.decorators import staff_member_required
+from django.contrib.auth.models import User
 from django.core.validators import validate_email
 from django.db.models import Max
 from django.http import Http404, HttpResponse
@@ -275,6 +276,10 @@ def email_settings(request, key):
         preferred_language = request.POST.get('preferred_language')
         validation_passed = True
         try:
+            email_in_use = User.objects.filter(email=email) | User.objects.filter(profile__email=email)
+            email_used_marketing = EmailSubscriber.objects.filter(email=email).select_related('profile')
+            if (email_in_use or email_used_marketing) and (request.user.email != email or request.user.profile.email != email):
+                raise ValueError(f'{request.user} attempting to use an email which is already in use on the platform')
             validate_email(email)
         except Exception as e:
             print(e)