Always query emails case-insensitively

src/pretix/base/forms/auth.py

@@ -120,7 +120,7 @@ class RegistrationForm(forms.Form):
 
     def clean_email(self):
         email = self.cleaned_data['email']
-        if User.objects.filter(email=email).exists():
+        if User.objects.filter(email__iexact=email).exists():
             raise forms.ValidationError(
                 self.error_messages['duplicate_email'],
                 code='duplicate_email'