Catch duplicate recipient columns in spreadsheets

If someone has duplicate recipient columns in their file we don’t know
which one to use. This commit adds an error message which should help
them fix the duplication.

This commit doesn’t go to the extra effort to actually show the
correct values for duplication in the preview. Don’t think it’s worth
the effort/complexity for how infrequently we’ve seen this error.

Depends on:
- [ ] https://github.com/alphagov/notifications-utils/pull/376

app/main/views/send.py

@@ -581,6 +581,7 @@ def check_messages(service_id, template_type, upload_id, row_index=2):
         data['recipients'].too_many_rows or
         not data['count_of_recipients'] or
         not data['recipients'].has_recipient_columns or
+        data['recipients'].duplicate_recipient_column_headers or
         data['recipients'].missing_column_headers
     ):
         return render_template('views/check/column-errors.html', **data)