diff --git a/app/aws/s3.py b/app/aws/s3.py
index b23cca71d..a440745da 100644
--- a/app/aws/s3.py
+++ b/app/aws/s3.py
@@ -4,6 +4,7 @@ from flask import current_app
 
 import pytz
 from boto3 import client, resource
+import botocore
 
 FILE_LOCATION_STRUCTURE = 'service-{}-notify/{}.csv'
 
@@ -18,6 +19,17 @@ def get_s3_object(bucket_name, file_location):
     return s3.Object(bucket_name, file_location)
 
 
+def file_exists(bucket_name, file_location):
+    try:
+        # try and access metadata of object
+        get_s3_object(bucket_name, file_location).metadata
+        return True
+    except botocore.exceptions.ClientError as e:
+        if e.response['ResponseMetadata']['HTTPStatusCode'] == 404:
+            return False
+        raise
+
+
 def get_job_location(service_id, job_id):
     return (
         current_app.config['CSV_UPLOAD_BUCKET_NAME'],
diff --git a/app/celery/research_mode_tasks.py b/app/celery/research_mode_tasks.py
index ac2e1e8dd..5fbe09b01 100644
--- a/app/celery/research_mode_tasks.py
+++ b/app/celery/research_mode_tasks.py
@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timedelta
 import json
 
 from flask import current_app
@@ -7,6 +7,7 @@ from requests import request, RequestException, HTTPError
 from notifications_utils.s3 import s3upload
 
 from app import notify_celery
+from app.aws.s3 import file_exists
 from app.models import SMS_TYPE
 from app.config import QueueNames
 from app.celery.process_ses_receipts_tasks import process_ses_results
@@ -123,7 +124,18 @@ def firetext_callback(notification_id, to):
 def create_fake_letter_response_file(self, reference):
     now = datetime.utcnow()
     dvla_response_data = '{}|Sent|0|Sorted'.format(reference)
-    upload_file_name = 'NOTIFY-{}-RSP.TXT'.format(now.strftime('%Y%m%d%H%M%S'))
+
+    # try and find a filename that hasn't been taken yet - going back in time 60 seconds
+    for i in range(30):
+        upload_file_name = 'NOTIFY-{}-RSP.TXT'.format((now - timedelta(seconds=i)).strftime('%Y%m%d%H%M%S'))
+        if not file_exists(current_app.config['DVLA_RESPONSE_BUCKET_NAME'], upload_file_name):
+            break
+    else:
+        raise ValueError(
+            'cant create fake letter response file for {} - too many files for that time already exist on s3'.format(
+                reference
+            )
+        )
 
     s3upload(
         filedata=dvla_response_data,
diff --git a/tests/app/celery/test_research_mode_tasks.py b/tests/app/celery/test_research_mode_tasks.py
index 77656a62e..aacfd81d5 100644
--- a/tests/app/celery/test_research_mode_tasks.py
+++ b/tests/app/celery/test_research_mode_tasks.py
@@ -1,5 +1,5 @@
 import uuid
-from unittest.mock import ANY
+from unittest.mock import ANY, call
 
 from flask import current_app, json
 from freezegun import freeze_time
@@ -111,6 +111,7 @@ def test_failure_firetext_callback(phone_number):
 @freeze_time("2018-01-25 14:00:00")
 def test_create_fake_letter_response_file_uploads_response_file_s3(
         notify_api, mocker):
+    mocker.patch('app.celery.research_mode_tasks.file_exists', return_value=False)
     mock_s3upload = mocker.patch('app.celery.research_mode_tasks.s3upload')
     filename = 'NOTIFY-20180125140000-RSP.TXT'
 
@@ -134,6 +135,7 @@ def test_create_fake_letter_response_file_uploads_response_file_s3(
 @freeze_time("2018-01-25 14:00:00")
 def test_create_fake_letter_response_file_calls_dvla_callback_on_development(
         notify_api, mocker):
+    mocker.patch('app.celery.research_mode_tasks.file_exists', return_value=False)
     mocker.patch('app.celery.research_mode_tasks.s3upload')
     filename = 'NOTIFY-20180125140000-RSP.TXT'
 
@@ -159,6 +161,7 @@ def test_create_fake_letter_response_file_calls_dvla_callback_on_development(
 @freeze_time("2018-01-25 14:00:00")
 def test_create_fake_letter_response_file_does_not_call_dvla_callback_on_preview(
         notify_api, mocker):
+    mocker.patch('app.celery.research_mode_tasks.file_exists', return_value=False)
     mocker.patch('app.celery.research_mode_tasks.s3upload')
 
     with set_config_values(notify_api, {
@@ -168,3 +171,35 @@ def test_create_fake_letter_response_file_does_not_call_dvla_callback_on_preview
             create_fake_letter_response_file('random-ref')
 
             assert request_mock.last_request is None
+
+
+@freeze_time("2018-01-25 14:00:30")
+def test_create_fake_letter_response_file_tries_to_create_files_with_other_filenames(notify_api, mocker):
+    mock_file_exists = mocker.patch('app.celery.research_mode_tasks.file_exists', side_effect=[True, True, False])
+    mock_s3upload = mocker.patch('app.celery.research_mode_tasks.s3upload')
+
+    create_fake_letter_response_file('random-ref')
+
+    assert mock_file_exists.mock_calls == [
+        call('test.notify.com-ftp', 'NOTIFY-20180125140030-RSP.TXT'),
+        call('test.notify.com-ftp', 'NOTIFY-20180125140029-RSP.TXT'),
+        call('test.notify.com-ftp', 'NOTIFY-20180125140028-RSP.TXT'),
+    ]
+    mock_s3upload.assert_called_once_with(
+        filedata=ANY,
+        region=ANY,
+        bucket_name=ANY,
+        file_location='NOTIFY-20180125140028-RSP.TXT'
+    )
+
+
+@freeze_time("2018-01-25 14:00:30")
+def test_create_fake_letter_response_file_gives_up_after_thirty_times(notify_api, mocker):
+    mock_file_exists = mocker.patch('app.celery.research_mode_tasks.file_exists', return_value=True)
+    mock_s3upload = mocker.patch('app.celery.research_mode_tasks.s3upload')
+
+    with pytest.raises(ValueError):
+        create_fake_letter_response_file('random-ref')
+
+    assert len(mock_file_exists.mock_calls) == 30
+    assert not mock_s3upload.called