Don't error sending a letter that's sent already

Fixes this error (in Admin):

      File "/home/vcap/app/app/notify_client/notification_api_client.py", line 74, in send_precompiled_letter
        return self.post(url='/service/{}/send-pdf-letter'.format(service_id), data=data)
      File "/home/vcap/app/app/notify_client/__init__.py", line 59, in post
        return super().post(*args, **kwargs)
      File "/home/vcap/deps/0/python/lib/python3.9/site-packages/notifications_python_client/base.py", line 48, in post
        return self.request("POST", url, data=data)
      File "/home/vcap/deps/0/python/lib/python3.9/site-packages/notifications_python_client/base.py", line 64, in request
        response = self._perform_request(method, url, kwargs)
      File "/home/vcap/deps/0/python/lib/python3.9/site-packages/notifications_python_client/base.py", line 118, in _perform_request
        raise api_error
    notifications_python_client.errors.HTTPError: 500 - Internal server error

Due to this error (in API):

      File "/home/vcap/app/app/service/send_notification.py", line 178, in send_pdf_letter_notification
        raise e
      File "/home/vcap/app/app/service/send_notification.py", line 173, in send_pdf_letter_notification
        letter = utils_s3download(current_app.config['TRANSIENT_UPLOADED_LETTERS'], file_location)
      File "/home/vcap/deps/0/python/lib/python3.9/site-packages/notifications_utils/s3.py", line 53, in s3download
        raise S3ObjectNotFound(error.response, error.operation_name)
    notifications_utils.s3.S3ObjectNotFound: An error occurred (NoSuchKey) when calling the GetObject operation: The specified key does not exist.

I checked the DB to verify the letter does actually exist i.e. it
is an instance of the problem we're fixing here.

app/service/send_notification.py

@@ -7,7 +7,10 @@ from sqlalchemy.orm.exc import NoResultFound
 
 from app import create_random_identifier
 from app.config import QueueNames
-from app.dao.notifications_dao import _update_notification_status
+from app.dao.notifications_dao import (
+    _update_notification_status,
+    get_notification_by_id,
+)
 from app.dao.service_email_reply_to_dao import dao_get_reply_to_by_id
 from app.dao.service_sms_sender_dao import dao_get_service_sms_senders_by_id
 from app.dao.services_dao import dao_fetch_service_by_id
@@ -172,9 +175,14 @@ def send_pdf_letter_notification(service_id, post_data):
     try:
         letter = utils_s3download(current_app.config['TRANSIENT_UPLOADED_LETTERS'], file_location)
     except S3ObjectNotFound as e:
-        current_app.logger.exception('Letter {}.pdf not in transient {} bucket'.format(
+        current_app.logger.warning('Letter {}.pdf not in transient {} bucket'.format(
             post_data['file_id'], current_app.config['TRANSIENT_UPLOADED_LETTERS'])
         )
+
+        # notification already exists e.g. if the user clicked send in different tabs
+        if get_notification_by_id(post_data['file_id']):
+            return {'id': str(post_data['file_id'])}
+
         raise e
 
     # Getting the page count won't raise an error since admin has already checked the PDF is valid

tests/app/service/test_send_pdf_letter_notification.py

@@ -75,14 +75,41 @@ def test_send_pdf_letter_notification_raises_error_when_pdf_is_not_in_transient_
     notify_user,
 ):
     user = sample_service_full_permissions.users[0]
-    post_data = {'filename': 'valid.pdf', 'created_by': user.id, 'file_id': fake_uuid, 'postage': 'first',
-                 'recipient_address': 'Bugs%20Bunny%0A123%20Main%20Street%0ALooney%20Town'}
+    post_data = {
+        'filename':
+        'valid.pdf',
+        'created_by': user.id,
+        'file_id': fake_uuid,
+        'postage': 'first',
+        'recipient_address': 'Bugs%20Bunny%0A123%20Main%20Street%0ALooney%20Town'
+    }
     mocker.patch('app.service.send_notification.utils_s3download', side_effect=S3ObjectNotFound({}, ''))
 
     with pytest.raises(S3ObjectNotFound):
         send_pdf_letter_notification(sample_service_full_permissions.id, post_data)
 
 
+def test_send_pdf_letter_notification_does_nothing_if_notification_already_exists(
+    mocker,
+    sample_service_full_permissions,
+    fake_uuid,
+    notify_user,
+    sample_notification,
+):
+    user = sample_service_full_permissions.users[0]
+    post_data = {
+        'filename':
+        'valid.pdf',
+        'created_by': user.id,
+        'file_id': str(sample_notification.id),
+        'postage': 'first',
+        'recipient_address': 'Bugs%20Bunny%0A123%20Main%20Street%0ALooney%20Town'
+    }
+    mocker.patch('app.service.send_notification.utils_s3download', side_effect=S3ObjectNotFound({}, ''))
+    response = send_pdf_letter_notification(sample_service_full_permissions.id, post_data)
+    assert response['id'] == str(sample_notification.id)
+
+
 @freeze_time("2019-08-02 11:00:00")
 def test_send_pdf_letter_notification_creates_notification_and_moves_letter(
     mocker,